Fix mailer to use webpack core

app/javascript/flavours/glitch/theme.yml

@@ -15,6 +15,7 @@ pack:
     - flavours/glitch/async/getting_started
     - flavours/glitch/async/home_timeline
     - flavours/glitch/async/notifications
+  mailer:
   modal:
   public: packs/public.js
   settings:

app/javascript/flavours/vanilla/theme.yml

@@ -15,6 +15,7 @@ pack:
     - features/compose
     - features/home_timeline
     - features/notifications
+  mailer:
   modal:
   public: public.js
   settings: