FEATURE: Support for localized themes (#6848)

- Themes can supply translation files in a format like `/locales/{locale}.yml`. These files should be valid YAML, with a single top level key equal to the locale being defined. For now these can only be defined using the `discourse_theme` CLI, importing a `.tar.gz`, or from a GIT repository.

- Fallback is handled on a global level (if the locale is not defined in the theme), as well as on individual keys (if some keys are missing from the selected interface language).

- Administrators can override individual keys on a per-theme basis in the /admin/customize/themes user interface.

- Theme developers should access defined translations using the new theme prefix variables:
  JavaScript: `I18n.t(themePrefix("my_translation_key"))`
  Handlebars: `{{theme-i18n "my_translation_key"}}` or `{{i18n (theme-prefix "my_translation_key")}}`

- To design for backwards compatibility, theme developers can check for the presence of the `themePrefix` variable in JavaScript

- As part of this, the old `{{themeSetting.setting_name}}` syntax is deprecated in favour of `{{theme-setting "setting_name"}}`

spec/lib/theme_javascript_compiler_spec.rb

@@ -0,0 +1,102 @@
+require 'rails_helper'
+
+require_dependency 'theme_javascript_compiler'
+
+describe ThemeJavascriptCompiler do
+
+  let(:theme_id) { 22 }
+
+  describe ThemeJavascriptCompiler::RawTemplatePrecompiler do
+    # For the raw templates, we can easily render them serverside, so let's do that
+
+    let(:compiler) { described_class.new(theme_id) }
+
+    let(:helpers) {
+      <<~JS
+      Handlebars.registerHelper('theme-prefix', function(themeId, string) {
+        return `theme_translations.${themeId}.${string}`
+      })
+      Handlebars.registerHelper('theme-i18n', function(themeId, string) {
+        return `translated(theme_translations.${themeId}.${string})`
+      })
+      Handlebars.registerHelper('theme-setting', function(themeId, string) {
+        return `setting(${themeId}:${string})`
+      })
+      Handlebars.registerHelper('dummy-helper', function(string) {
+        return `dummy(${string})`
+      })
+      JS
+    }
+
+    let(:mini_racer) {
+      ctx = MiniRacer::Context.new
+      ctx.eval(File.open("#{Rails.root}/vendor/assets/javascripts/handlebars.js").read)
+      ctx.eval(helpers)
+      ctx
+    }
+
+    def render(template)
+      compiled = compiler.compile(template)
+      mini_racer.eval "Handlebars.template(#{compiled.squish})({})"
+    end
+
+    it 'adds the theme id to the helpers' do
+      # Works normally
+      expect(render("{{theme-prefix 'translation_key'}}")).
+        to eq('theme_translations.22.translation_key')
+      expect(render("{{theme-i18n 'translation_key'}}")).
+        to eq('translated(theme_translations.22.translation_key)')
+      expect(render("{{theme-setting 'setting_key'}}")).
+        to eq('setting(22:setting_key)')
+
+      # Works when used inside other statements
+      expect(render("{{dummy-helper (theme-prefix 'translation_key')}}")).
+        to eq('dummy(theme_translations.22.translation_key)')
+    end
+
+    it 'works with the old settings syntax' do
+      expect(render("{{themeSetting.setting_key}}")).
+        to eq('setting(22:setting_key)')
+
+      # Works when used inside other statements
+      expect(render("{{dummy-helper themeSetting.setting_key}}")).
+        to eq('dummy(setting(22:setting_key))')
+    end
+  end
+
+  describe ThemeJavascriptCompiler::EmberTemplatePrecompiler do
+    # For the Ember (Glimmer) templates, serverside rendering is not trivial,
+    # so check the compiled JSON against known working output
+    let(:compiler) { described_class.new(theme_id) }
+
+    def statement(template)
+      compiled = compiler.compile(template)
+      data = JSON.parse(compiled)
+      block = JSON.parse(data["block"])
+      block["statements"]
+    end
+
+    it 'adds the theme id to the helpers' do
+      expect(statement("{{theme-prefix 'translation_key'}}")).
+        to eq([[1, [27, "theme-prefix", [22, "translation_key"], nil], false]])
+      expect(statement("{{theme-i18n 'translation_key'}}")).
+        to eq([[1, [27, "theme-i18n", [22, "translation_key"], nil], false]])
+      expect(statement("{{theme-setting 'setting_key'}}")).
+        to eq([[1, [27, "theme-setting", [22, "setting_key"], nil], false]])
+
+      # Works when used inside other statements
+      expect(statement("{{dummy-helper (theme-prefix 'translation_key')}}")).
+        to eq([[1, [27, "dummy-helper", [[27, "theme-prefix", [22, "translation_key"], nil]], nil], false]])
+    end
+
+    it 'works with the old settings syntax' do
+      expect(statement("{{themeSetting.setting_key}}")).
+        to eq([[1, [27, "theme-setting", [22, "setting_key"], [["deprecated"], [true]]], false]])
+
+      # Works when used inside other statements
+      expect(statement("{{dummy-helper themeSetting.setting_key}}")).
+        to eq([[1, [27, "dummy-helper", [[27, "theme-setting", [22, "setting_key"], [["deprecated"], [true]]]], nil], false]])
+    end
+  end
+
+end