Allow choosing custom versions of python for graders

[JasonGrace2282]

Motivation

• Sets us up for supporting Java
• Allows us to offer higher versions of python, without breaking backwards compatibility with graders relying on python 3.10 behavior

Note that fields which may be useful for implementing e.g. sandboxed Java ARE NOT included in this change. I would rather add those changes in a separate PR, and keep the diff here smaller.

Implementation

This adds a new model, Language, which stores information about the language, binary path, and more. The field Assignment.language has been moved inside said model.

Important

Custom migrations were written in order to facilitate this transition with minimal problems.

The rest is just some restructuring:

• Changes the run_submission task to use the assignments executable
• Filters the edit assignment/create venv to only show non-deprecated language options.
• Changes the create_venv task to use the correct executable, and

TODO

Stuff to do until this PR is ready to review (feel free to help!)

• Add Language model
• Write custom migrations to handle data changes
• Figure out why unmigrating fails
• Update assignment editing UI to reflect this
• Change internal tasks to use the correct language/binary paths
• Figure out how to handle java paths & python language grader