Move websocket-transport to its own repo

#9831 (moved) created /pluggable-transports/websocket.git. Let's move /flashproxy.git/websocket-transport there.

If I remember correctly, the procedure goes something like

1. Clone your local flashproxy repo and put it in a directory called websocket.
2. In the new websocket directory, run

git filter-branch --subdirectory-filter websocket-transport -- --all

(Check the man page for git-filter-branch.) 3. Check the new history and make sure it looks right and makes sense. It might be worth doing git log -C -M --stat websocket-transport in the flashproxy repo to check if any files might have lived in another directory at some point. 4. Add a remote that points to the new /pluggable-transports/websocket.git. 5. Push to the new remote.

I think it makes sense to move doc/websocket-transport.txt into the new repo as well, and update references to it in the flashproxy source code.