BUG Escape or sanitize assignment names

[nickelpro]

Describe the bug
Assignment names containing invalid characters for a GH repo name cause infinite problems. Discovered when I dared to put a colon in an assignment name, "Homework 1: Subtitle"

To Reproduce
Steps to reproduce the behavior:

1. Create an assignment
2. Change the assignment name to something containing an invalid character, like a colon
3. Try to create a repo
4. Everything burns

Expected behavior
Either an error when trying to save an invalid name, or a sanitization routine applied to assignment names prior to creating repos with them.

Screenshots
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Additional context
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[github-actions]

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