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/local/set-environment

@@ -0,0 +1,29 @@
+#!/bin/sh
+
+enabled="$(snapctl get enabled)"
+if [ -z "${enabled}" -o "${enabled}" != "true" ]
+then
+    echo "Plume not yet enabled"
+    exit 1
+fi
+
+export BASE_URL="$(snapctl get base-url)"
+database_type="$(snapctl get db.type)"
+
+if [ z"${database_type}" = z"sqlite" ]
+then
+    export DATABASE_URL=${SNAP_DATA}/plume.db
+    export MIGRATION_DIR=migrations/sqlite
+else
+    # Must be postgres, so must have set db.url
+    export DATABASE_URL="$(snapctl get db.url)"
+    export MIGRATION_DIRECTORY=migrations/postgres
+fi
+
+ROCKET_ADDRESS="$(snapctl get listen.address)"
+ROCKET_PORT="$(snapctl get listen.port)"
+export ROCKET_SECRET_KEY="$(cat ${SNAP_COMMON}/rocket-secret-key)"
+export SEARCH_INDEX="${SNAP_DATA}/search_index"
+
+cd ${SNAP}
+exec $@