Cannot accept project invite if email is mixed-case

**Describe the bug**

- Invite a user by entering their email address with mixed-case, e.g. `User.Lorem@example.com`
- After the user signed up, they cannot accept the invitation.

This is because the "accounts" app is case-insensitive by default and the server found `User.Lorem@example.com` not matched with `user.lorem@example.com`

Start by tracing the accounts app's project-invitation acceptance flow and the email lookup used after signup. Reproduce the mixed-case invitation scenario, then add coverage for matching User.Lorem@example.com with user.lorem@example.com. Done means invited users can accept invitations regardless of email casing.

Written by the indexing model from the issue text.