AppView: allows a default knot to be configured #857

This follows on from the work carried out in #836

I've added a select box in the Knots settings page which pulls in the users knots and also adds in knot1.tangled.sh. When the user selects one of these options, it will save to their profile in the database. NOTE: I haven't yet implemented adding that to the AT record because I'm not sure on how the lexicon setup works yet!

Then when users go to create a new repo / fork, if there is a value in their profile for the default knot, then that will pre select the knot to use for the new repo / fork.