Reserve username when hellbanning trouble user

Fix https://gitlab.com/gitterHQ/webapp/-/issues/2708
7 jobs for !2117 with 2708-fix-user-circumventing-ban in 20 minutes and 32 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Build Unit Test
passed #945890358
gitlab-org
package

00:06:31

passed #945890356
gitlab-org
test

00:11:44

manual #945890357
internal allowed to fail manual
test_e2e
passed #945890355
gitlab-org
validate

00:05:06

 
  Pre Deploy
passed #945890359
gitlab-org
distribute_beta_staging

00:00:40

 
  Deploy
passed #945890360
gitlab-org-docker
containerize

00:08:07

manual #945890361
internal allowed to fail manual
deploy_beta_staging