Various snap fixes (#698)

* snap: Store media in the proper directory

* snap/set-environment: Exit successfully when not enabled.

This lets us unconditionally run things under `set-environment` and let it
handle the case where the necessary configuration hasn't yet been set.

* snap: Set the version based on git tags

This will make the version something like `0.3.0-alpha-2-65-g29178aa` at the moment,
as that was the most recent tag, but the commit tagged as `0.4.0` will get *that* as
the version

* snap: Force-install cargo-web

This makes building the *second* time work; otherwise there might be
an existing cargo-web install, and cargo will refuse to overwrite it.

snap/hooks/configure

@@ -38,6 +38,11 @@ then
 with 'snap restart plume'"
 fi
 
+if [ ! -d ${SNAP_DATA}/media ]
+then
+    mkdir ${SNAP_DATA}/media
+fi
+
 if [ "${enabled}" = "true" -a ! -e ${SNAP_COMMON}/initial-migrations-run ]
 then
     cd ${SNAP}