Release workflow for Firefox Lite

All development happens on master.

There is an internal, invitation-only Nightly that builds every day against previewWebkitRelease flavor.

When there's a release, we release through focusWebkitRelease. If master contains some commits that we don't want for this release, we create another branch to pick whatever we want for this release and name it release/a.b.c. Otherwise, we use master directly and add a tag a.b.c.