Fixes webhook, adds fabric script to manage it

This fixes the webhook once and for all, by moving the hookshot daemon's "working directory" out of the `$HOME/hub/current` symlink and into `$HOME`. Previously, the webhook would work for one deploy, but then fail on the _second_ deploy because its previous current working directory had been destroyed.

To reduce the chances of this happening again, I've also adapted the `fabric` tasks from the [18f.gsa.gov deploy process](https://github.com/18F/18f.gsa.gov/tree/staging/deploy), meaning that there's now a `deploy/fabfile.py` in addition to the project's root `fabfile.py`. The new fabric file contains `start`, `stop`, and `restart` tasks for the webhook itself, and makes sure that it's run out of the home directory.

To run these new hookshot fabric tasks, you'll need to have Python 2.7.X activated, and be running `fabric` out of the `deploy/` directory.

Fixes #249.
I moved the `deploy-site.sh` script from a one-off unversioned thing on the server, to a versioned location inside the repo. This has a lot of clear benefits, but it does mean that the version of the `deploy-site.sh` command that the webhook runs is dependent on the version of the script placed there in the *previous* deploy.