Feature/support branches

This PR adds in the 'support branches' feature:

* Support branches are for maintaining old versions of your app whilst develop & master are tracking the latest release
* Support branches are created from the master branch
* You can make releases from a support branch; when finished, they get merged back into the support branch
* You can make hotfixes from a support branch; when finished, they get merged into both the support branch and develop

It's an invasive change, so use with caution for now.