New files encryption app

The new files encryption app is ready to be merged into master.

Known  shortcomings:

- If a file was shared to a group and a new user gets added to the group the file will  not be encrypted automatically to the user. 
- Upgrade old encryption -> new encryption in master doesn't work because the initial upgrade process doesn't migrate the complete file cache which makes it impossible for the encryption app to discover files encrypted with the old encryption during first log-in, see https://github.com/owncloud/core/issues/3415
- If the user loses his password and the recovery key feature was enabled the admin can set a new password and all files in the users root folder will be re-encrypted with the new user key. But the user will lose access to files shared with him since they doesn't get re-encrypted at the moment.

Some stuff you should know:

If recovery mode is enabled, both in the admin settings and in the personal user settings. Than the files will be recovered if the admin change the user password in the user settings if he provides the recovery password along the new user password. If this feature is not enabled or if the admin doesn't provide a recovery password while he changes the user password all files from the given user will be lost.