Add username filtering

[xXPinkmagicXx]

When a user creates a new account, they will have the option to choose their username.
This introduces some problems.

1. Do we want to reserve some usernames?
   • Admin, System, Moderator
   • Zeeguu
   • Other?
2. Do we want to filter/ban some words in usernames?
   • Swear words (in all languages) ?
   • Do we want both backend and frontend?

Note: We should also think about normalization of characters as users can bypass filters with:

• Cyrillic lookalikes
• Accents
• Zero-width characters

[xXPinkmagicXx]

Possible solutions for (2.)

1. (js) https://www.npmjs.com/package/leo-profanity
2. (py) https://pypi.org/project/better-profanity/
3. (py) unitary/multilingual-toxic-xlm-roberta (transformer classification model)

What solution we go with? @mircealungu @gabortodor @klnyzzz33

[xXPinkmagicXx]

This is still related to the #910, as you can edit you profile info and username.