Add :registrations_recaptcha feature flag

What does this MR do?

• Adds :registrations_recaptcha feature flag as guard clause to recaptcha load and verification. enabled by default

• Moves flag guard, original guards (incl. verification) and error path to check_recaptcha action callback.

Maybe now an opportunity to DRY SessionsController, the original implementer of a check_recaptcha callback. I wanted to leave this to BE because I was starting to think down the route of a RecaptchaActions controller concern for reuse. But not 100% on that and the details. v. happy to open an issue if BE reviewers agree this needs follow up.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• 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

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