Use a method for creating groups in the API

EE has some custom behaviour that needs to run before and after creating
a group in the API. By moving the group creation logic to a separate
method, EE can easily extend this behaviour; without having to directly
modify the API source code.

lib/api/groups.rb

@@ -58,6 +58,14 @@ module API
       end
       # rubocop: enable CodeReuse/ActiveRecord
+      def create_group
+        # This is a separate method so that EE can extend its behaviour, without
+        # having to modify this code directly.
+        ::Groups::CreateService
+          .new(current_user, declared_params(include_missing: false))
+          .execute
+      end
+
       def find_group_projects(params)
         group = find_group!(params[:id])
         options = {
@@ -127,7 +135,7 @@ module API
           authorize! :create_group
         end
-        group = ::Groups::CreateService.new(current_user, declared_params(include_missing: false)).execute
+        group = create_group
         if group.persisted?
           present group, with: Entities::GroupDetail, current_user: current_user