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.

app/helpers/qunit_helper.rb

@@ -2,7 +2,7 @@
 
 module QunitHelper
   def theme_tests
-    theme = Theme.find_by(id: request.env[:resolved_theme_ids]&.first)
+    theme = Theme.find_by(id: request.env[:resolved_theme_id])
     return "" if theme.blank?
 
     _, digest = theme.baked_js_tests_with_digest