Disable login button while awaiting backend response

Currently the login button is not disabled during the login process. As a result of a current issue with the LDAP servers at ELTE FI (in production), the authentication can take multiple seconds and sometimes it fails for some reason (which to be investigated by the system administrators of the faculty).

As a side effect, many users click the login button repeatedly, often around 10 times in a short interval. This results in multiple login endpoint calls on the backend and the backend indeed issues multiple authentication requests towards the LDAP serves. As a result, both TMS backend and the LDAP servers get a significant extra load, further escalating the original problem.

This should be resolved by disabling the login button until the response from the backend arrives; and for a minimum 2 seconds anyway, in case an error is returned immediately. Maybe meanwhile the login button could also display the information that logging in is in progress.