Upgrade to FontAwesome 5 (take two) (#6673)

* Add missing icons to set

* Revert FA5 revert

 This reverts commit 42572ff

* use new SVG syntax in locales

* Noscript page changes (remove login button, center "powered by" footer text)

* Cast wider net for SVG icons in settings

- include any _icon setting for SVG registry (offers better support for plugin settings)

- let themes store multiple pipe-delimited icons in a setting

- also replaces broken onebox image icon with SVG reference in cooked post processor

* interpolate icons in locales

* Fix composer whisper icon alignment

* Add support for stacked icons

* SECURITY: enforce hostname to match discourse hostname

This ensures that the hostname rails uses for various helpers always matches
the Discourse hostname

* load SVG sprite with pre-initializers

* FIX: enable caching on SVG sprites

* PERF: use JSONP for SVG sprites so they are served from CDN

This avoids needing to deal with CORS for loading of the SVG

Note, added the svg- prefix to the filename so we can quickly tell in
dev tools what the file is

* Add missing SVG sprite JSONP script to CSP

* Upgrade to FA 5.5.0

* Add support for all FA4.7 icons

- adds complete frontend and backend for renamed FA4.7 icons

- improves performance of SvgSprite.bundle and SvgSprite.all_icons

* Fix group avatar flair preview

- adds an endpoint at /svg-sprites/search/:keyword

- adds frontend ajax call that pulls icon in avatar flair preview even when it is not in subset

* Remove FA 4.7 font files

lib/discourse_plugin_registry.rb

@@ -15,6 +15,7 @@ class DiscoursePluginRegistry
     attr_writer :handlebars
     attr_writer :serialized_current_user_fields
     attr_writer :seed_data
+    attr_writer :svg_icons
     attr_writer :locales
     attr_accessor :custom_html
 
@@ -59,6 +60,10 @@ class DiscoursePluginRegistry
       @sass_variables ||= Set.new
     end
 
+    def svg_icons
+      @svg_icons ||= []
+    end
+
     def handlebars
       @handlebars ||= Set.new
     end
@@ -105,6 +110,10 @@ class DiscoursePluginRegistry
     self.service_workers << filename
   end
 
+  def self.register_svg_icon(icon)
+    self.svg_icons << icon
+  end
+
   def register_css(filename)
     self.class.stylesheets << filename
   end