recover encryption keys if ownCloud can't change the log-in password

This patch enables the admin to recover the users files even if it is not possible to update the users password through ownCloud.

In this case two steps are necessary (it doesn't matter in which order you perform both steps):

- change the log-in password at the back-end (e.g. LDAP)

- go to ownClouds user management. Insert the recovery password to the appropriated filed (I added a tool-tip for a better understanding of the input field) and change the password from the relevant user. -> you will get a message that the back-end doesn't support password change from within ownCloud but that the encryption key was updated successfully)

@FlorinPeter please review the code
@blizzz would be great if you could give it a try with a real LDAP back-end