[encryption] create backup from all keys before recovery

Steps to test:

1. create two users "admin" and "user"
2. "admin" enables encryption
3. "admin" enabled the recovery key in the admin settings
4. "user" enables recovery key in the personal settings
5. "user" uploads some files
6. "admin" changes the password from "user" in the user settings with the recovery pasword
7. look at ```/data/user/files_encryption```, you should find a ```backup.recovery.<date>``` folder which contains all file keys, share keys and the users public key.
8. login as "user", you should be able to read the files with the new key -> logout again
9. copy filekeys, share-keys and the private-key from the backup folder back to ```data/user2/files_encryption``` and the public key from the backup to ```data/public-keys```
10. login again as "user" -> you should get a warning that login password doesn't match key password
11 follow the instruction and update the private key in the personal settings by providing the old login password (before recovery) and the new login password (set by the admin during recovery)
12. Now you should be able to read the files again with the backup keys.

cc @PVince81 

fix #11061
can we please have a unit test for this? THX