allow labels to be used on Pull Requests to configure the auto_run flags on contexts

[jstrachan]

it would be nice to be able to create a Pull Request but disable automatically triggering a context until its explicitly manually triggered. Or maybe to enable a context which isn't automatically triggered.

e.g. creating a PR on the version stream from lighthouse, it'd be nice to enable most git provider BDD tests for that PR only.

Another use case is being able to trigger a promote PR but disable the validation check until folks are ready to run it (rather than doing it automatically); ditto for BDD tests on the version stream; it would be nice to create PRs but disable the auto-run so that they can be ran later.

Label notation

We generally want to be able to:

• enable or disable all contexts
• enable/disable specific named contexts

Would be nice to come up with a simple polymorphic label convention for those things?

e.g.

• lighthouse/auto_run/off turn all contexts off
• lighthouse/auto_run/on turn all contexts on
• lighthouse/auto_run/off/foo turn off auto_run for foo
• lighthouse/auto_run/on/bar turn off auto_run for bar