Moves Protected paths throttling from Omnibus to GitLab Rails

What does this MR do?

Moves protected paths to GitLab-Rails:

• Adds 4 columns to application_settings
  • 3 to mimic the configuration of existing throttles
  • 1 to store the protected paths on database
• Set default protected paths (taken from Omnibus)
• Add new section on admin panel to personalize protected paths configuration
• This new throttle is active by default.
• Includes additional protected paths throttles
• Rack Attack file was renamed to rack_attack_new.rb, otherwise the Omnibus file will overwrite this file.
• If the Omnibus settings are present, the application settings are ignored.

Related to #29952 (closed)

Documentation MR - !16540 (merged)

Screenshots

With Omnibus throttle present	Without Omnibus throttle

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32773
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

To do - backend

• Move Protected Paths to config/gitlab.yml
• Handle the throttle on RackAttackGlobal
• Add specs
• Log error on auth.log
• Add new application_settings to UI
• Refactor
• Configure protected paths on UI