add indexes to user model

[itsthejoker]

Relevant issue: N/a

Description:

Login queries take a while because the email field isn't indexed; we also use the username field to filter by elsewhere, so it doesn't hurt to throw an index there too.

Checklist:

• [x] Code Quality
• [x] Pep-8
• [ ] Tests (if applicable)
• [x] Success Criteria Met
• [ ] Inline Documentation
• [ ] Wiki Documentation (if applicable)