Automatically stop nightly containers when starting stable, and vice versa

[kdmccormick]

We discussed elsewhere why this would be difficult, but this still happens enough to me that I'd like it in the backlog as an issue, even if we never get to it.

[kdmccormick]

I implemented this as a plugin: https://github.com/kdmccormick/tutor-contrib-kdmccormick#stopnightly
Source: https://github.com/kdmccormick/tutor-contrib-kdmccormick/blob/master/tutorkdmccormick/stopnightly.py

Unfortunately the plugin is hacky in a couple ways:

• it assumes that __app__ is always "tutor" or "tutor-nightly", which is not guaranteed
• it reaches into several tutor modules outside of the supported tutor.hooks API.