Changes to Git branching strategy

[ralf-berger]

Initiated by #1275, the branching strategy of the project changes from basically GitHub flow (or Feature branch flow) to a variant of Git Flow, but without hotfix, release branches and any versioning.

[ralf-berger]

[ralf-berger]

The more involved branching strategy introduced by rawaa123/shoebjoarder is not being adopted as intended:

• I'm contributing to dev now, but PRs (Update main with accumulated changes to the integration branch #1311) from dev are not being reviewed/merged into main at all.
• At the same time, PRs from feature branches ([webapp][webserver] Dialog is shown when a material is uploaded to te… #1345, [webapp] preview slides has been enhanced !! #1346) are being approved and merged by rawaa123 directly into main, bypassing dev completely.