Centralize all LDAP config logic in `GitLab::LDAP::Config`. Previously,
some logic was in the Devise initializer and it was not honoring the
`user_filter`. If a user outside the configured `user_filter` signed
in, an account would be created but they would then be denied access.
Now that logic is centralized, the filter is honored and users outside
the filter are never created.
It was previously possible for invalid credential errors to go unnoticed
in this task. Users would believe everything was configured correctly and
then sign in would fail with 'invalid credentials'. This adds a specific
bind check, plus catches errors connecting to the server. Also, specs :)
