Clean up the branches in the Github repo

[abias]

The set of branches in this Github repo is not as clean as it should be.

The target set is like this:

• There is a main branch which contains the latest development code. This code must not be broken but it does not need to be production-ready
• There isn't a master branch anymore
• There is a MOODLE_405_STABLE branch which contains the latest release for Moodle 4.5 (which is published on moodle.org)
• At least the latest commit in the MOODLE_405_STABLE branch is tagges as v4.5-r (where x is the number of the release)
• There aren't any feature / issue / upgrade branches unless the code in this branch is still work in progress and there is a corresponding Github issue.