When adding or updating a custom user field to apply to all users (retroactively) we want to alert the admin that this will force all existing users to fill up the field before they are able to access the forum again.
However, we currently show this prompt when making changes only to other attributes on the custom field, i.e. the requirement hasn't changed.
This commit fixes that.
We want to allow admins to make new required fields apply to existing users. In order for this to work we need to have a way to make those users fill up the fields on their next page load. This is very similar to how adding a 2FA requirement post-fact works. Users will be redirected to a page where they can fill up the remaining required fields, and until they do that they won't be able to do anything else.
We're planning to implement a feature that allows adding required fields for existing users. This PR does some preparatory refactoring to make that possible. There should be no changes to existing behaviour. Just a small update to the admin UI.
