* Add Snapcraft metadata and install/maintenance hooks
* Move set-environment script into snap/local
* snap: Remove diesel-cli part.
This *was* necessary as of 0.3.0, but now `plm migrations run` does the job.
* snap: Add an actual description
* snap: Add an 'enabled' config item.
Don't try to run until enabled is set, and automatically run the
migrations when we are enabled
