add method to check if users private/public key exists
show change password form if key couldn't be decrypted
only create new key on password change if a recovery key exists or if the user don't have any private/public keys
fix typo
Merge pull request #6208 from owncloud/enc_only_create_new_key_if_no_other_key_exists

[Encryption] Only create new key if no other key exists