[encryption] reorganize folder structure

In order to fix #11297 I changed the folder structure for the encryption keys.

We create for every file a corresponding folder in files_encryption/keys. For example for "foo/test.txt" we will create a folder "files_encryption/keys/foo/test.txt/". This folder will contain all share keys, named "userId.shareKey" and the file key named "fileKey". This way we avoid any name collision with file names which are similar to user names, like described in issue #11297.

This makes many file operation easier. If a file gets renamed or moved we just have to rename/move the corresponding folder. This way we can save a lot of overhead to find the correct share-keys and file-keys which should result in some performance improvements and should also make the code more robust.

Additionally this PR changes the naming for public/private keys from "user.public.key" and user.private.key" to "user.publicKey" and user.privateKey" for consistency reasons.

This PR touches all aspects of encryption, so we need to test encryption as complete as possible, including:

- sharing, reshares,...
- deleting single files or complete folders
- restore the deleted file/folder from the trashbin or just single files/sub-folders from the deleted folder
- enabling recovery keys
- change passwords
- recover files
- deleted shared files and try to  restore them
- ...

Currently the migration script is missing. But everything should work if you start with a fresh installation or a setup without any encrypted files and encryption keys.

Currently missing:

- [ ] migration script
OCA\Encryption\Migration; will not be detected by the autoloader - this would require to add this class to the classpath variable. Usage of that classpath shall be deprecated some day - see https://github.com/owncloud/core/issues/11003

I'd welcome if the namespaces would be properly adjusted - THX