Invitation App should be displayed in Applications Panel only for admins by default

Permissions are set incorrectly. This was supposed to be fixed in XWIKI-11138 but there are errors in the permissions that need to be fixed.

Note that the idea here is to:

• Hide the Invitation entry from non admins
• Still allow normal users and even guests to access the Invitation home page since it needs to be accessible for the invitation feature to work