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/helpers/application_helper_spec.rb

@@ -89,7 +89,7 @@ describe ApplicationHelper do
           user_id: -1,
           color_scheme_id: ColorScheme.find_by(base_scheme_id: "Dark").id
         )
-        helper.request.env[:resolved_theme_ids] = [dark_theme.id]
+        helper.request.env[:resolved_theme_id] = dark_theme.id
       end
       context "on desktop" do
         before do
@@ -509,7 +509,7 @@ describe ApplicationHelper do
         user_id: -1,
         color_scheme_id: ColorScheme.find_by(base_scheme_id: "Dark").id
       )
-      helper.request.env[:resolved_theme_ids] = [dark_theme.id]
+      helper.request.env[:resolved_theme_id] = dark_theme.id
 
       expect(helper.dark_color_scheme?).to eq(true)
     end