Invite button should be disabled for rooms not allowing invites

[sefsh]

I dislike that a user may begin creating an invite and only afterwards get notified it's not allowed, if permissions restrict it for the current room.

Steps to reproduce

• Be in a room that doesn't allow you to create room invites
• Try to invite someone anyway
• Get frustrated since button indicated you could use it

I'd like for the button to be in a disabled state (if not hidden) for rooms that won't allow user-created invites.

Version information

Riot 1.1.0.