Signed-off-by: NRémy Coutable <remy@rymai.me>

It was using name, instead of username. Fixes documentation as well

Closes #46454

Also include a class method for retriving the gitlab_deploy_token on
DeployTokens

Also:
- Includes more specs
- Improves a bit the documentation

- If a deploy token with a name 'gitlab-deploy-token' is exists for the
  project, CI_DEPLOY_USER and CI_DEPLOY_PASSWORD variables will be
expose

Previously we scheduled a worker to just some this but we were running
into performance issues when the build table was getting too large. So
now we've updated the code such that this column is updated immediately
and incremented/decremented by the correct amount whenever artifacts are
created or deleted.

We've also added the performance optimization that we do not update this
statistic if a project is deleted because it could result in many
updates for a project with many builds.

The purpose of this memoization is to make getting persisted environment
name, and related scoped variables, a little more performant task,
because it can be invoked multiple times.