This PR (WIP) sets up some Gitflow processes.

Some requirements that need to be done in order for this to work/make sense:

• Extend all branch protection settings we have for master to develop (so have them for both branches)
• Make develop the default branch in Github
• Either make sure that approval of the "github-actions" bot user is OK to merge a PR, or we need to provide a github token of a user in whose name the approvals will be in

How it should work (overview)

• We develop on the new develop branch. Any PRs are merged into develop
• When we want to do a release, we merge develop into master and trigger a release from there.
• While a release is pending, PRs can still be merged into develop (they will just not go into the ongoing release anymore)

How to do a release going forward?

• Create a meta changelog PR to develop (as before)
• When this is merged, we'll automatically create a PR from develop->master, auto-approve it, and automerge it when CI passed
• Trigger a release from master branch (as before)
• When this is merged, we'll automatically create a PR from master->develop, auto-approve it, and automerge it when CI passed
• Now everything should be in sync again!

Potential streamlining/improvement?

One way in which this could be improved, is to auto-trigger the release whenever we merge into master.
In order to do this, we'll need to somehow know at this point:

• the version to release
• force if there are release blockers (optional)

We may be able to parse this somehow, but I'd say this is an area for future improvement, if we decide so.

What's missing?

We'll probably have to tweak caching etc. settings a bit to streamline this - e.g. what needs to run when merging develop-> master, what needs to run when merging master->develop, etc.