Reorganize branches

[MrSimbax]

• a new stable branch will have the same role as the old master branch
• a new main branch will have the same role as the old dev branch
• the main branch will be the default repository branch
• the dev- branches will contain big features to which multiple people can contribute at the same time, which means force pushes are not allowed to them
• the wip- branches will be personal branches, they can be forced pushed so the single person working on the branch can push whatever they want there and not care about commit messages, their content etc. as long as the history is cleaned up before opening a pull request to one of the other branches
• the fix- branches are for hot fixes and can only be based on main or stable branch
• other branch prefixes will be considered on a case-by-case basis, for example diversity- for a feature mainly for a competition named Diversity
• the master and dev branches shall remain frozen for the next 2 releases, in order to give time for everyone to update their local repositories and forks; after this period the branches will be deleted

Note: Besides settings on GitHub, scripts for CI and tools like release.py will also have to be updated.