Warn for password reset when files_encryption is enabled

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 :dancers:

Fixes https://github.com/owncloud/core/issues/11438
Fix scrutinizer issues
Remove uneeded import
Use correct route instead

THX @schiesbn
(I should setup a mail server on my local system...)
Add "postPasswordReset" hook
add password as parameter to the signal so that the encryption can create a new key-pair
listen to the post_passwordReset hook, backup the old keys and create a new key pair for the user
Hide submit button after password change

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.
fix unreadable label in warning box
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Merge pull request #11696 from owncloud/addWarningToEncryptionLostPassword

Warn for password reset when files_encryption is enabled