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/content_security_policy/middleware.rb

@@ -17,10 +17,10 @@ class ContentSecurityPolicy
       protocol = (SiteSetting.force_https || request.ssl?) ? "https://" : "http://"
       base_url = protocol + request.host_with_port + Discourse.base_path
 
-      theme_ids = env[:resolved_theme_ids]
+      theme_id = env[:resolved_theme_id]
 
-      headers['Content-Security-Policy'] = policy(theme_ids, base_url: base_url, path_info: env["PATH_INFO"]) if SiteSetting.content_security_policy
-      headers['Content-Security-Policy-Report-Only'] = policy(theme_ids, base_url: base_url, path_info: env["PATH_INFO"]) if SiteSetting.content_security_policy_report_only
+      headers['Content-Security-Policy'] = policy(theme_id, base_url: base_url, path_info: env["PATH_INFO"]) if SiteSetting.content_security_policy
+      headers['Content-Security-Policy-Report-Only'] = policy(theme_id, base_url: base_url, path_info: env["PATH_INFO"]) if SiteSetting.content_security_policy_report_only
 
       response
     end