FEATURE: Load theme setting descriptions from theme locale files

Previously theme setting descriptions were defined in the `settings.yml` file like this:
```
setting_name:
  default: "My Default Value"
  description:
    en: "English description"
    fr: "French description"
```

This commit allows developers to store the localised descriptions in the theme locale files instead:
```
en:
  theme_metadata:
    description: Theme Description
    settings:
      setting_name: "The localised description for setting_name"
```

app/models/theme.rb

@@ -367,7 +367,7 @@ class Theme < ActiveRecord::Base
   end
 
   def internal_translations
-    translations(internal: true)
+    @internal_translations ||= translations(internal: true)
   end
 
   def translations(internal: false)

app/serializers/theme_settings_serializer.rb

@@ -21,7 +21,8 @@ class ThemeSettingsSerializer < ApplicationSerializer
   end
 
   def description
-    object.description
+    locale_file_description = object.theme.internal_translations.find  { |t| t.key == "theme_metadata.settings.#{setting}" } &.value
+    locale_file_description || object.description
   end
 
   def valid_values
@@ -33,7 +34,7 @@ class ThemeSettingsSerializer < ApplicationSerializer
   end
 
   def include_description?
-    object.description.present?
+    description.present?
   end
 
   def list_type

lib/theme_settings_manager.rb

@@ -34,7 +34,7 @@ class ThemeSettingsManager
   end
 
   def description
-    @opts[:description]
+    @opts[:description] # Old method of specifying description. Is now overridden by locale file
   end
 
   def value=(new_value)