Change branch structure and release flow

[pheuberger]

What is it?

We want to move dev -> main and main -> release to fix our git semantics where main is the trunk where all work happens and release is what production points to.

We need two new GH actions:

1. on creating a PR into release builds the frontend and runs the unit test suite, to make sure the release PR is at least somewhat sound.
2. on merging the PR into main creates a PR to integrate main into release which will trigger a deployment when merged

[pheuberger]

We decided to not move ahead with renaming the branches just yet.

I will create a new PR to amend the workflow that creates a new publish-to-production PR to main instead of release.

[pheuberger]

Tracked in #480