* 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.
df6f398efc