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.

lib/middleware/anonymous_cache.rb

@@ -132,9 +132,9 @@ module Middleware
 
       def theme_ids
         ids, _ = @request.cookies['theme_ids']&.split('|')
-        ids = ids&.split(",")&.map(&:to_i)
-        if ids && Guardian.new.allow_themes?(ids)
-          Theme.transform_ids(ids)
+        id = ids&.split(",")&.map(&:to_i)&.first
+        if id && Guardian.new.allow_themes?([id])
+          Theme.transform_ids(id)
         else
           []
         end