Add some sort of notification that you have invitations to own a crate

[carols10cents]

@natboehm implemented the last big chunk of inviting users to be owners rather than just adding them!

Now there's a menu item in the user menu for "Owner invites", which lists pending invitations for you to accept or decline.

@natboehm also had a great idea that we should have some sort of notification that you have invitations to accept... I'm not sure how to do that though!!!