Continuous staging

[jeremydw]

Users should be able to add Git remotes to projects, and specify continuous build rules. Users should be able to link Git branches to preview domains so that when a commit occurs on the configured branch in the remote, the build should kick off and the build server should run a deployment command that deploys the build from the branch to a preview domain in Jetway.

[jeremydw]

There should also be a way to trigger a rebuild by pressing a button in the UI. Useful if a datasource (like a Google Sheet) changes.