Move reusable Identity Verification controller code to a base controller

Currently users go through identity verification process as part of the
registration process. Moving forward, we want to allow users that have
already started using Gitlab to go through identity verification to
complete verification methods–phone number and/or credit card
verification–that they didn't go through during registration in exchange
for in-app benefits (e.g. more CI minutes).

Here, we move controller code that are only relevant to identity
verification during registration to a new
RegistrationsIdentityVerificationController module and move all reusable
controller code from IdentityVerificationController to a new
BaseIdentityVerificationController module.