Centralize LDAP config/filter logic

What does this MR do?

Centralized all LDAP config logic in to Gitlab::LDAP::Config. We had varying configuration for devise/omniauth and other things. For example, user_filter was never taken in to account for devise/omniauth so a user object would always be created, even if the user did not match the user_filter.

Fixes gitlab-org/gitlab-ce#21195, https://gitlab.com/gitlab-org/gitlab-ce/issues/15396 and gitlab-org/gitlab-ce#13296

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

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