Improve invitation workflow

Our current invitation workflow has some glitches.

Currently you need to enter the E-Mailadress of a person you want to invite to your channel. Now, if that user got 2 or more emailadresses it might be somewhat difficult: The user might already have an account and gets an invitation to his second emailadress. Additionally you have to know the emailadresses. That's bad.

To improve that workflow, we should use chosen (http://harvesthq.github.com/chosen/) and its suggestion feature to suggest the user some user names of users, who are assigned to channels, which the current user is assigned to.

That allows users to invite people without knowing their emailadress.

Additionally we should provide an "invite to" chosen field with channels in the user profiles.

chosen-rails gem: https://github.com/tsechingho/chosen-rails

What if two users have the same name?
-> We should display the names like "name@channel". Should be enough