Show delete-own-messages action for group admins

[redactdeveloper]

Summary

• allow the AyuGram "Delete own messages" menu action for group and megagroup admins
• keep the existing restrictions that the peer must be a group/megagroup and the current user must still be a member

The previous check excluded creators and users with admin rights, which made the action disappear for admins as reported in #359.

Fixes #359.

Verification

• Ran git diff --check
• Full AyuGram Desktop build was not run locally because this checkout does not include the full dependency/build environment.