provider.json is downloaded before ca.crt

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.
Merge branch 'bug/get-ca-cert-url-from-provider-dot-json' into develop
Service is down message correctly shown.

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.
Merge branch 'bug/missing-error-message-when-web-api-is-down-#3378' into develop