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

@jancborchardt Requires some UI love.

\cc @gig13