Add Snapcraft metadata and install/maintenance hooks (#666)

* 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

snap/hooks/post-refresh

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+db_type=$(snapctl get db.type)
+
+if [ -z "${db_type}" ]
+then
+    exit 0
+fi
+
+cd ${SNAP}
+exec ./set-environment bin/plm migration run --path ${SNAP_DATA}