Fix broken Django unit tests due to previous commits

• Commit efe2287a removed legacy answers and payments, breaking tests
• Commit 7b387f12 allowed changing category names without affecting slug, breaking tests
• Commit 82ba785b removed wiki transfer memos, breaking tests
• Move to ediauth from myauth broke tests relying on display name being different from username

This MR fixes all of them, all 69 tests pass.