Models: fix username case-insensitive comparator & release: v5.0.1

[ptamarit]

Fixes https://github.com/zenodo/ops/issues/326

❤️ Thank you for your contribution!

Description

Usernames are enforced to be unique in the DB regardless of their case (see related DB column and constraint definition).

This change proposes to compare usernames case-insensitively.
This will avoid problems like users trying to sign up with an existing username but with a different case, which are currently hitting the DB constraint and then showing an nginx 502 error since it is not well handled by Flask.

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• I'm aware of the code of conduct.
• I've created logical separate commits and followed the commit message format.
• I've added relevant test cases.
• I've added relevant documentation.
• I've marked translation strings (for relevant code).
• I've followed the CSS/JS and React guidelines (for relevant code).
• I've followed the web accessibility guidelines (for relevant code).
• I've followed the user interface guidelines (for relevant code).
• I've identified the copyright holder(s) and updated copyright headers for touched files (>15 lines contributions).
• I've NOT included third-party code (copy/pasted source code or new dependencies).

Third-party code

If you've added third-party code (copy/pasted or new dependencies), please reach out to an architect.

Reminder

By using GitHub, you have already agreed to the GitHub’s Terms of Service including that:

1. You license your contribution under the same terms as the current repository’s license.
2. You agree that you have the right to license your contribution under the current repository’s license.