Rewrite admin Encryption manual page

[carlaschroder]

Calling @LukasReschke and all security gurus-- please review.

[LukasReschke]

Add a notice that this list is possible not finished. i.e. other files may be unencrypted as well and only the files that are exposed possibly to third-party storage providers are guaranteed to be encrypted.

(i.e. third-party app data etc.)

[LukasReschke]

Also @schiesbn as he mainly maintains the encryption app.

[LukasReschke]

(Will be only available on my mobile until Monday - a first look looks good. Will take a look then again.)

[schiessle]

Added a note about the folder for the encryption keys of system wide external storages, see my commit above.

One note regarding LDAP and other user backends at the bottom of the page: Admins can change the login password on the LDAP or on another back-end. As far as I know that's also how most ownCloud setups are maintained with LDAP. If this happens the user will see a warning the next time he logs in that the login password no longer matches the private key password. The user can then adjust the private key password in his personal settings by providing his old login password and the new one. Just try it out, maybe you can also make some screenshots from it and add it to the documentation.
(EDIT: Just saw that it is already mentioned in the user documentation: https://github.com/owncloud/documentation/blob/encryption-user/user_manual/files/encryption.rst, but still it might be useful to mention it also in the admin documentation)

Regarding the initial encryption, maybe it is worth mentioning that with respect to shared files we can only encrypt the files to recipients who already have a private key. If the recipient of a share hasn't login before the shared file will not be encrypted for him. The next time he tries to access the shared file he will get a warning and the suggestion that he should ask the owner to re-share the file with him. Recommend action for the owner:

If it is a user share: unshare it and share it again
If it is a group share: keep the group share and just share the file/folder additionally to the user who can't access the file. The owner can immediately remove the single user share again. The encryption was updated and the recipient should be able to access the files again.

[carlaschroder]

I call it good. 👍 Thanks Lukas and Björn.