Prevent channels with same name

[tzarebczan]

Right now, this validation is only happening on the create form. So if you create a channel, and then go back, you can create another channel with the same name. We've seen a few instances of these on spee.ch, even days apart (not sure how that happens yet). When logging in, it would not know which channel to login to (I believe it uses the earliest one).

[alyssaoc]

adding as part of channel ux