Make two factor concern module more auto-load friendly

What does this MR do?

Make concern module more auto-load friendly.

The existing folder app/controllers/admin/concerns is not part of the top-level auto-load and hence, making EE extension for this concern is problematic. Other non-admin controllers (e.g. OmniauthCallbacksController) are also using this concern so it is better placed in the standard app/controllers/concerns folder.

The class is also renamed to be consistent with the existing counterpart AuthenticatesWithTwoFactor class and avoid issue with RSpec cop misreading the name due to usages of numbers and acronyms (i.e. 2FA).

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability 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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team