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/snapcraft.yaml

@@ -1,6 +1,6 @@
 name: plume
 base: core18
-version: '0.3.0' # just for humans, typically '1.2+git' or '1.3.2'
+adopt-info: plume
 summary: Multi-user blogging platform, federated over ActivityPub
 description: |
   Plume is a federated blogging platform, featuring:
@@ -34,9 +34,10 @@ parts:
       - on arm64,armhf,ppc64el,s390x:
         - lld-8
     override-build: |
+      snapcraftctl set-version $(git describe --tags)
       export PATH=$PATH:$HOME/.cargo/bin
       rustup install stable
-      cargo +stable install cargo-web
+      cargo +stable install --force cargo-web
 
       # Only Tier 1 Rust platforms get rust-lld
       # On the others (arm64, armhf, powerpc64, s390x) fall back to using