Remove all assumptions about default branch names

[LinuxMercedes]

Assigner hails from the days when hardwiring "master" as the default branch name made sense, but that is fortunately no longer the case. We should be able to fetch the default branch from Gitlab.

Questions:

• Should we check the default branch for each repo, or assume that student repos use the same default branch as the template repo they're made from?

Docs:

• https://git.mst.edu/help/user/project/repository/branches/default.md
• https://git.mst.edu/help/api/projects.md
• https://git.mst.edu/help/api/branches.md

[BehindTheBrain]

I implemented and checked this task.
All branch name assumptions were removed.
I think that explicit is better than implicit default detection.
Thus, in this fork, --branch is required for a specified set of sub-commands.

https://github.com/BehindTheBrain/assigner

Going forward, I will likely tend to simplify and remove complexity, rather than add non-critical features.
So, whether I maintain a separate fork, or take over as a maintainer, is up to y'all.