Usage limits 2fa fixes

[LMNTL]

Checklist

1. If you've added code that should be tested, add tests
2. If you've changed APIs, update (or create!) the documentation
3. Ensure the tests pass
4. Make sure that your code lints and that you've followed our coding style
5. Write a description of your work suitable for publishing on our forum
6. Mention any related issues in this repository (as #ISSUE) and in other repositories (as kobotoolbox/other#ISSUE)
7. Open an issue in the docs if there are UI/UX changes

Description

Small changes to how MFA configuration is sent to the browser.

Notes

The frontend environment variable mfa_allowed has been split into three values:

• mfa_allowed: tracks whether MFA is enabled on the server level
• mfa_per_user_availability: tracks whether the current user has MFA per-user availability set
• mfa_has_availability_list: tracks whether any users have MFA per-user availability set

[noliveleger]

This condition does not work when MFA is enabled and there is not per-user availability list. MFA section never shows up.
It could be changed to isMfaAvailable: !hasMfaList || (hasMfaList && mfaAvailableToUser),