Username management command + allow unicode usernames and make username read only

[ahmed-arb]

We have to allow usernames as received from Wikimedia (with Unicode characters) and make them read-only at the time of account creation.
Also, migrate the already existing management command to update the previously cleaned usernames with their original usernames (on Wikimedia)

PRs:

• fix: made username readonly #568
• chore: enabled unicode usernames tutor-contrib-wikilearn#13