Add channels to profile table

This adds a channels row to the profile table.

The idea is to give users a place to list their other channels which we can then use in other places - such as the profile_vcard, on the permissions page for that channel, etc

I'm pull requesting this before making it actually do anything because I have doubts about whether it's a good idea or not.  A technically better solution would be to find all the channels associated with an account, and add a new permission to decide who can see which, but since people are already complaining that everything is complicated, this might be easier all round.