Did not plan to work on git-annex today..

Unexpectedly ended up making the webapp support HTTPS. Not by default,
but if a key and certificate are provided, it'll use them. Great for
using the webapp remotely! See the new tip: [[tips/remote_webapp_setup]].

Also removed support for --listen with a port, which was buggy and not
necessary with HTTPS.

Also fixed several webapp/assistant bugs, including one that let it be run in
a bare git repository.

And, made the quvi version be probed at runtime, rather than compile time.