Prevent changing MFA to ui_only

[kevinlinxc]

Addresses the remaining half of #2968 (flash messages already resolved/being resolved)

Hides ui_only option for MFA, unless the user is currently using ui_only, ultimately preventing people from switching to ui_only.

Testing:
MFA set to something else:

Set MFA to ui_only:

Result:

[kevinlinxc]

Considered making a less DRY version, maybe make a list, and insert at position 2 if mfa_ui_only? Let me know if that would be better. Alright I just did it

[jchestershopify]

Code looks fine, but I think we should add a test as there is some logic added.

[jenshenny]

Looking good! Along with tests, it'll be great to add a check in the backend to make sure they aren't updating to ui_only (possibly adding a flash banner if someone tries to).

[kevinlinxc]

Added backend code, flash banner, and tests. Had to refactor a bit because of codeclimate

[jenshenny]

A few suggestions that should be addressed but otherwise LGTM! Could you also refactor/squash your commits?
Also, I think an update to the guides would be needed 🤔

[jenshenny]

✨ 🚀

@sonalkr132 are we ok with merging and deprecating ui_only right now (with the only heads up being an update to the guides). I think it would be ok as a small percentage are at this level + we are now recommending ui_and_gem_signin and ui_and_api.

[simi]

🙏 please squash commits before merge

[kevinlinxc]

Rebased and squashed

[kevinlinxc]

Went with Updating multi-factor authentication to "UI Only" is no longer supported. Please use "UI and gem signin" or "UI and API". RubyGems latest failing though

[sonalkr132]

Thank you @kevinlinxc