PERF: Eager load Theme associations in Stylesheet Manager.

Before this change, calling `StyleSheet::Manager.stylesheet_details`
for the first time resulted in multiple queries to the database. This is
because the code was modelled in a way where each `Theme` was loaded
from the database one at a time.

This PR restructures the code such that it allows us to load all the
theme records in a single query. It also allows us to eager load the
required associations upfront. In order to achieve this, I removed the
support of loading multiple themes per request. It was initially added
to support user selectable theme components but the feature was never
completed and abandoned because it wasn't a feature that we thought was
worth building.

spec/components/scss_checker_spec.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe Stylesheet::Manager::ScssChecker do
+  fab!(:theme) { Fabricate(:theme) }
+
+  describe '#has_scss' do
+    it 'should return true when theme has scss' do
+      scss_theme = Fabricate(:theme, component: true)
+      scss_theme.set_field(target: :common, name: "scss", value: ".scss{color: red;}")
+      scss_theme.save!
+
+      embedded_scss_theme = Fabricate(:theme, component: true)
+      embedded_scss_theme.set_field(target: :common, name: "embedded_scss", value: ".scss{color: red;}")
+      embedded_scss_theme.save!
+
+      theme_ids = [scss_theme.id, embedded_scss_theme.id]
+
+      desktop_theme_checker = described_class.new(:desktop_theme, theme_ids)
+
+      expect(desktop_theme_checker.has_scss(scss_theme.id)).to eq(true)
+      expect(desktop_theme_checker.has_scss(embedded_scss_theme.id)).to eq(false)
+
+      embedded_theme_checker = described_class.new(:embedded_theme, theme_ids)
+
+      expect(embedded_theme_checker.has_scss(scss_theme.id)).to eq(false)
+      expect(embedded_theme_checker.has_scss(embedded_scss_theme.id)).to eq(true)
+    end
+
+    it 'should return false when theme does not have scss' do
+      expect(described_class.new(:desktop_theme, [theme.id]).has_scss(theme.id))
+        .to eq(false)
+    end
+  end
+end