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/lib/theme_flag_modifier_spec.rb

@@ -10,7 +10,7 @@ describe ThemeModifierHelper do
   end
 
   it "can extract theme ids from a request object" do
-    request = Rack::Request.new({ resolved_theme_ids: [theme.id] })
+    request = Rack::Request.new({ resolved_theme_id: theme.id })
     tmh = ThemeModifierHelper.new(request: request)
     expect(tmh.serialize_topic_excerpts).to eq(true)
   end