Allow choosing custom versions of python for graders

[JasonGrace2282]

Motivation

• Sets us up for supporting Java (notably this DOES NOT add official Java support)
• Allows us to offer higher versions of python, without breaking backwards compatibility with graders relying on python 3.10 behavior

This PR also does not add a warning banner if an assignment is using a deprecated version of a language, in order to reduce the already large and complex diff. This would be better as a "good first issue".

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. All the information about the specific language is stored in a dictionary called info.

The info dictionary must contain a version key that is sortable. For the python grader language, it must also contain the key python3 which has the absolute path to a python executable.
The addition of another language to Tin can then consist of simply adding other keys to the info dictionary as needed.

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