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/local/set-environment

@@ -4,7 +4,7 @@ enabled="$(snapctl get enabled)"
 if [ -z "${enabled}" -o "${enabled}" != "true" ]
 then
     echo "Plume not yet enabled"
-    exit 1
+    exit 0
 fi
 
 export BASE_URL="$(snapctl get base-url)"
@@ -24,6 +24,7 @@ ROCKET_ADDRESS="$(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"
+export MEDIA_UPLOAD_DIRECTORY="${SNAP_DATA}/media"
 
 cd ${SNAP}
 exec $@