Relieve @PVince from having to write it all the time ;-)

If the webroot has been set to an empty value or ownCloud has been installed at the root location (`/``) there is a fair chance that the redirect for password resets does not work at all.

This means that while the password is getting resetted the user is not redirected to the login page.

I'm aware that it might be better to just set the webroot to `/` in those cases but this patch is better in the regard that it cannot break stuff.

Thanks to @PVince81 for helping me debugging this. (I'm a moron and assumed it couldn't be THAT easy)

Reported by @cdamken

For apps that support async translation loading, a new function
OC.L10N.load() can be used to asynchronously load the translations
for a given app.

Creating a new key pair can take 1-2 seconds. So it could happen that the user click the "Reset password" button again which can lead to many nasty things, e.g. we could create two new key pairs in parallel.

THX @schiesbn
(I should setup a mail server on my local system...)

This patch wil warn the user of the consequences when resetting the password and requires checking a checkbox (as we had in the past) to reset a password.

Furthermore I updated the code to use our new classes and added some unit tests for it 

Fixes https://github.com/owncloud/core/issues/11438

fixes download issue introduced by #10755

Conflicts:
	apps/files_sharing/public.php

Fixes https://github.com/owncloud/core/issues/12047