webapp: Adds newly created repositories to one of these groups:
clients, drives, servers
This is heuristic, but it's a pretty good heuristic, and can always be
configured.
Both when queueing downloads, and uploads, consults the preferred content
settings.
I didn't make it check yet when requeing failed transfers or queuing
deferred downloads; dealing with the preferred content settings (or indeed,
other settings) changing while the assistant is running still needs work.
