Update branch's commit statuses based on a non-PR bot

[czechboy0]

Think about how we could also add a syncer for not just PRs, but only for a "monitored" branch, e.g. master. A way to monitor a branch with a bot and to update the commit's status based on the bot's result. Would require a slight refactor where we'd allow the syncer to instead of resolving (PR + Bot) pairs to (branch + Bot) pairs, where PRs would just be one source of the monitored branch and we might have the user manually specify another, non-PR'ed branch to also be monitored and updated on GitHub.

• Split the main syncer into multiple classes
• Migrate to a SyncPair process, where each pair (Bot + PR, No Bot + PR, Bot + No PR) has its own class that handles its actions. Will scale better for new syncable pairs (Bot + Branch, ...)
• Add tests for the right SyncPair creation based on inputs from GitHub and XCS
• Introduce a Bot + Branch Sync Pair, which will share a lot with Bot + PR, share as much as possible
• Add a way in the UI to mark particular branches to be synced permanently (no need for a PR)
• Answer the question: will we allow picking a different Build Template for synced branches?
• Enable this on master here in Buildasaur and test that it works well for a couple of days

[czechboy0]

Work started.

[czechboy0]

At the moment, we use the PR's Build Template. If this is a problem and you'd like to pick a different template for different branches, create an issue with the description of your usecase and I'll see what I can do.