Take more declarative approach with membership actions

- clearer code and less re-branching.

app/controllers/concerns/membership_actions.rb

@@ -140,39 +140,15 @@ module MembershipActions
   end
 
   def root_params_key
-    case membershipable
-    when Namespace
-      :group_member
-    when Project
-      :project_member
-    else
-      raise "Unknown membershipable type: #{membershipable}!"
-    end
+    raise NotImplementedError
   end
 
   def members_page_url
-    case membershipable
-    when Namespace
-      polymorphic_url([membershipable, :members])
-    when Project
-      project_project_members_path(membershipable)
-    else
-      raise "Unknown membershipable type: #{membershipable}!"
-    end
+    raise NotImplementedError
   end
 
   def source_type
-    @source_type ||=
-      begin
-        case membershipable
-        when Namespace
-          _("group")
-        when Project
-          _("project")
-        else
-          raise "Unknown membershipable type: #{membershipable}!"
-        end
-      end
+    raise NotImplementedError
   end
 
   def plain_source_type

app/controllers/groups/group_members_controller.rb

@@ -82,6 +82,18 @@ class Groups::GroupMembersController < Groups::ApplicationController
   def plain_source_type
     'group'
   end
+
+  def source_type
+    _("group")
+  end
+
+  def members_page_url
+    polymorphic_url([group, :members])
+  end
+
+  def root_params_key
+    :group_member
+  end
 end
 
 Groups::GroupMembersController.prepend_mod_with('Groups::GroupMembersController')

app/controllers/projects/project_members_controller.rb

@@ -66,6 +66,18 @@ class Projects::ProjectMembersController < Projects::ApplicationController
   def plain_source_type
     'project'
   end
+
+  def source_type
+    _("project")
+  end
+
+  def members_page_url
+    project_project_members_path(project)
+  end
+
+  def root_params_key
+    :project_member
+  end
 end
 
 Projects::ProjectMembersController.prepend_mod_with('Projects::ProjectMembersController')