Add ability to change an instance default theme from the administration panel (#7092) (#8381)

* Add default_settings class method to ScopedSettings

ScopedSettings was extended to use value of unscoped setting instead of
only using defaults set in config/settings.yml for selected settings.
This adds possibility for admins to set default values of users' settings,
for example default theme (as requested in #7092).

* Add ability to change an instance default theme

Closes #7092

app/lib/settings/scoped_settings.rb

@@ -2,6 +2,10 @@
 
 module Settings
   class ScopedSettings
+    DEFAULTING_TO_UNSCOPED = %w(
+      theme
+    ).freeze
+
     def initialize(object)
       @object = object
     end
@@ -50,15 +54,22 @@ module Settings
       Rails.cache.fetch(Setting.cache_key(key, @object)) do
         db_val = thing_scoped.find_by(var: key.to_s)
         if db_val
-          default_value = Setting.default_settings[key]
+          default_value = ScopedSettings.default_settings[key]
           return default_value.with_indifferent_access.merge!(db_val.value) if default_value.is_a?(Hash)
           db_val.value
         else
-          Setting.default_settings[key]
+          ScopedSettings.default_settings[key]
         end
       end
     end
 
+    class << self
+      def default_settings
+        defaulting = DEFAULTING_TO_UNSCOPED.map { |k| [k, Setting[k]] }.to_h
+        Setting.default_settings.merge!(defaulting)
+      end
+    end
+
     protected
 
     def thing_scoped