Centralize LDAP config/filter logic

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.