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

Some stuff you should know:

If recovery mode is enabled, both in the admin settings and in the personal user settings. 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.