DEV: Use the theme cache helper for settings

The previous Discourse.cache usage was different to how other theme-related caching is handled, and also requires reaching out to redis every time. The common theme cache is held in memory (as a DistributedCache)

app/models/theme.rb

@@ -91,7 +91,6 @@ class Theme < ActiveRecord::Base
     update_javascript_cache!
 
     remove_from_cache!
-    clear_cached_settings!
     DB.after_commit { ColorScheme.hex_cache.clear }
     notify_theme_change(with_scheme: notify_with_scheme)
 
@@ -135,7 +134,6 @@ class Theme < ActiveRecord::Base
 
   after_destroy do
     remove_from_cache!
-    clear_cached_settings!
     if SiteSetting.default_theme_id == self.id
       Theme.clear_default!
     end
@@ -529,13 +527,13 @@ class Theme < ActiveRecord::Base
   end
 
   def cached_settings
-    Discourse.cache.fetch("settings_for_theme_#{self.id}", expires_in: 30.minutes) do
+    Theme.get_set_cache "settings_for_theme_#{self.id}" do
       build_settings_hash
     end
   end
 
   def cached_default_settings
-    Discourse.cache.fetch("default_settings_for_theme_#{self.id}", expires_in: 30.minutes) do
+    Theme.get_set_cache "default_settings_for_theme_#{self.id}" do
       settings_hash = {}
       self.settings.each do |setting|
         settings_hash[setting.name] = setting.default
@@ -561,13 +559,6 @@ class Theme < ActiveRecord::Base
     hash
   end
 
-  def clear_cached_settings!
-    DB.after_commit do
-      Discourse.cache.delete("settings_for_theme_#{self.id}")
-      Discourse.cache.delete("default_settings_for_theme_#{self.id}")
-    end
-  end
-
   def included_settings
     hash = {}
 

app/models/theme_field.rb

@@ -378,7 +378,6 @@ class ThemeField < ActiveRecord::Base
       DB.after_commit { Stylesheet::Manager.clear_theme_cache! }
     elsif settings_field?
       validate_yaml!
-      theme.clear_cached_settings!
       DB.after_commit { CSP::Extension.clear_theme_extensions_cache! }
       DB.after_commit { SvgSprite.expire_cache }
       self.value_baked = "baked"

app/models/theme_translation_override.rb

@@ -5,7 +5,6 @@ class ThemeTranslationOverride < ActiveRecord::Base
 
   after_commit do
     theme.theme_fields.where(target_id: Theme.targets[:translations]).update_all(value_baked: nil)
-    theme.clear_cached_settings!
     theme.remove_from_cache!
   end
 end

spec/models/theme_spec.rb

@@ -512,7 +512,7 @@ HTML
     theme.save!
 
     Upload.find(upload.id).destroy
-    theme.clear_cached_settings!
+    theme.remove_from_cache!
 
     json = JSON.parse(cached_settings(theme.id))
     expect(json).to be_empty