feat(build): prefer releasing on tag to releasing on merge to master

The former was set up in order to avoid having release branches, but
produces more problems than it solves if multiple people are
contributing to the repository. Instead, only build on tagged release
versions, whatever branch they happen to be in.

In order to ensure random PRs don't trigger releases, tag protection has
now been added to the repo, allowing only admins/maintainers to push
version tags.

.github/workflows/push-master.yaml → .github/workflows/release.yaml

@@ -2,8 +2,8 @@ name: Build and Deploy
 
 on:
    push:
-      branches:
-         - master
+      tags:
+         - 'v[0-9]+.[0-9]+.[0-9]+'
 
 jobs:
    build-and-deploy: