Grace Period field is not disabled on load when MFA is set to Optional

[Cheddam]

Tested version: 4.1.x-dev

Outline

MFA adds some configuration in the Access tab of the Settings area, covering whether MFA is Optional or Required, and if Required, when the requirement should be enforced (the 'grace period'.) The grace period field should be disabled if MFA is set to Optional, and this is triggered if the setting is switched to Optional, but not if it is already set to Optional when loading the Settings UI.

#346 fixed a similar issue, but seems not to have covered this case.

Steps to reproduce

1. Have MFA set to Optional in the Settings -> Access UI
2. Reload the CMS, and open the Settings -> Access UI
3. Observe that the Grace Period field is editable