add invite feature to user interface

This enables the invitation page also on the user interface.

The page allows selection between "all public domains" or a selection dialog of domains the user has explicit permissions to create invites for.

If a user only has the "all public domains" permission, then no selection is shown.

The invites can be either for restricted user names or "admin", which includes names such as info@.

The invitation dialog is not quite perfect. If a user has :user invite permission for some domains and :admin permissions for others, then he can request an invitation combination that is not valid and that the backend will reject. But it seems overly complicated (for users, but also the implementation) to add a dialog that models this properly.

depends on iapi!1 (closed)

I am more than happy to rebase ontop of !8 (closed) if that one is merged soonish