Fixed notifications too (forgot to do it in another branch, it's #5964).

It's enough to reflect the fact that the button leads to a dialog from
which the user can both log in or sign up.

It takes for granted that the certificate is valid from the very same
date it's downloaded.

I've also fixed another message: if a file to be downloaded is not
found, we say the entered provider isn't a LEAP provider. This is based
on the fact that if provider.json doesn't exist/contain valid
information, then the url isn't pointing to a LEAP provider.

We don't assume ca.crt is in /ca.crt anymore, but fetch the complete url
from provider.json.

We also signup against users.json file instead of simple "users", which
worked for *.bitmask.net domains.