Update Vagrantfile for Salt deployment

* Add salt provider
* Track develop branch of salt for bootstrap. This addresses an issue with
  postgres_user present in 0.17. Pinning the next stable release should be
  adequate
* Bump up memory
* Change web port to match nginx config
* Add better dns resolver, maybe not required
Migrate Chef config to Salt, improve site deploy

* Migrated most services 1-to-1
* Updated package install to work with pip 1.5+
* Updated application setup to perform actions on database as well
* Added temporary local_settings to get the server running
Add hiredis requirement
Add possible mentors to the GSOC page
Always update our ssh keys on users update.
Bump theme.
Turn on RTD canonical URL creation.
Clean up Project apiv2, and add django-filtering.
Clean up symlink API code
Handle sorting of slug based on sanity. This should really be easier.
Initial support for mkdocs markdown building
Initial support for mkdocs
Add nice version_to_slug util, and pull management command.
Update builds.rst

Pretty sure after 3 years other doc builders are still not on the horizon.
Update webhooks.rst

The repo admin page is now called "Settings."
backends/git: Simplified fallback branch handling
backends/git: Removed duplicate "git fetch" call

"git fetch -t" is fetching *only* the tags, while "git fetch" is fetching
branches *and* tags already.
backends/git: Extracted repo_url_changed() method
backends/git: Extracted repo_exists() method
backends/git: Extracted local revision variable