Add support for 2FA in admin mode feature

It was agreed that 2FA is not a hard requirement for a first GA release of the admin mode feature. Moving this to its own issue.

@reprazent @ifarkas @jbroullon

/cc @bufferoverflow

The following discussion from !18214 (merged) should be addressed:

• @dlouzan started a discussion: (+5 comments)

  @reprazent @ifarkas I've been doing some tests today and I'm not sure if it will be possible to add 2FA support without customizing / duplicating the 2FA views, something I'd like to avoid, at least in this MR.

  I basically incorporated some admin mode code in AuthenticatesWithTwoFactor#authenticate_with_two_factor, but although I'm able to provoke the 2FA code input in the flow, I wasn't able to trigger the authenticate_with_two_factor method above in the re-authentication. As far as I can see this is only triggered on SessionsController:

  class SessionsController < Devise::SessionsController
    ...
      prepend_before_action :authenticate_with_two_factor,
        if: -> { action_name == 'create' && two_factor_enabled? }
    ...

  I guess we could incorporate this too in a dedicated method for Admin::SessionsController, but for that I understand I'd need to be able to customize the endpoint called by the 2FA form (hence my statement above about needing to duplicate the 2FA views).

  Thoughts?