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/requests/safe_mode_controller_spec.rb

@@ -11,6 +11,8 @@ RSpec.describe SafeModeController do
       theme.set_default!
 
       get '/safe-mode'
+
+      expect(response.status).to eq(200)
       expect(response.body).not_to include("My Custom Header")
     end
   end