Bruteforce timeout via public capabilities / Show warning if login is delayed by bruteforce #1178

As proposed here nextcloud/server#4510 (comment)
and implemented here nextcloud/server#5567 we will get the possibility to ask for the current bruteforce timeout.
As we are switching to weblogin flow for >= nc12 and I enhanced the bruteforce support for all version (cookie is now sent with every request, so only first request ever will take long) I doubt if we need this.

With the timeout we could show a little warning if the login is delayed by bruteforce.