Enable the login button only when exam has started

[Nimon77]

Prevent user from logging in to exam session before exam is started (may be 42Paris specific)

[FreekBes]

The _enableOrDisableSubmitButton is intended to enable the submit button for the form (so essentially the login button) already, there's no need to create an extra function for it. The reasoning behind this is to have all the logic that enables or disables the button in the same function for every possible login/lock screen.

Could you make it so that the _enableOrDisableSubmitButton checks the current time, and enables the button when it needs to be?

Side note: I did this once before, thought of it as a small change and didn't test it properly, and found out during an exam the function for some reason didn't work. So I strongly recommend to test it during a test exam before, even though the logic might appear solid.

I've made the changes myself