Assignments Fixes

[Clashsoft]

• OpenAI and Moss settings are properly saved again (introduced in Restructure Assignment OpenAI and Moss Config #430)

Improvements

• When loading the list of students during GitHub Import fails, a toast now informs about the problem (e.g. invalid token).
• Improved success messages for Import actions.
• When no assignees exist in an assignment, menus in the solution table now properly react.

Bugfixes

• Editing the OpenAI model option now works correctly.
• Fixed z-fighting in solution table.
• Fixed some internal issues related to embedding imports.