feat(front,back): remove field "userName"

[spy4x]

Reason:

it's rarely used by SaaS startups, mostly by social networks.

Parts:

1. Backend endpoint @Get('is-username-free/:userName')
2. DTO UserIsUsernameFreeDTO
3. Query UserIsUsernameFreeQuery
4. QueryHandler and its usages UserIsUsernameFreeQueryHandler
5. Field definition in Prisma schema file
6. Prisma Migration for the new field. Create with $ yarn prisma migrate dev --name users_add_email
7. Any other usages of wording "username", "userName" and "USERNAME" (there are places on backend and frontend, but mostly single-line removals

Testing:

• Check that command $ yarn format && yarn precommit && yarn nx affected:build succeeds
• Check that new user can be created in app front-web (http://localhost:4200)

Related to https://github.com/spy4x/seed/issues/109 - field "email" replaces field "userName".