Better AutoFreezer PR continuation

Problem

I let KSP-CKAN/NetKAN#9471 sit without review for quite a while, and in the meantime we had 9 weekly checks go by without any new commits pushed. Probably quite a few abandoned mods have slipped through the cracks, which we should probably catch up at some point.

Cause

The branch checkout logic in the AutoFreezer was trying to re-use an existing branch if there was one, but it was missing a step: it fetched the remote branch, and it tried to check it out, but it never created a local branch corresponding to the remote branch. So checking it out failed, and it always fell back to creating a new branch. Then later the attempt to push to the server would fail because the branch already existed with conflicts.

Changes

Now the AutoFreezer uses the shared NetkanRepo.change_branch function instead of rolling its own version of that logic, which will fix the problem and reduce code duplication
To help with review, the bot will now post its intended PR body as a comment when it pushes commits to an existing open PR. This way the person reviewing will see the table of the newly added mods with their last update and links.

This way if we miss a week of review, the next week's idle mods will get cleanly added on to the existing PR, and none will be missed.

Note that this branch shares the test-fixes commit with #280, but not the one with that PR's actual functionality. I think we can merge either one first or not at all and the other one should be fine to continue or not.

KSP-CKAN / NetKAN-Infra