Allow regex pattern for expected student files.

[james-perretta]

This would be in addition to shell-style wildcards.

[james-perretta]

Proposed design:

• Enum with 3 options: exact match, shell wildcard, or regex.
• When exact match, use ==
• When shell wildcard, use fnmatch
• When regex, use re.match

Note that this will need a custom migration in order to add the new enum field. We can use the same auto-detect logic from the frontend code to populate it.

[james-perretta]

There hasn't been much demand for this. Closing unless it gets requested more.