Fixes WebDAV copy

Append search results to custom container

Add async command system to handle asynchronous operations

Remove dependency from arbitrary data object structure for easier usage

Remove uneeded assignment

Failing mysql master tests

Clean up AppFramework database layer

using Doctrine\DBAL\Connection::executeUpdate()

Show 404 page when accessing empty share URL

Return null when requesting tags for null user

Remove unused and deprecated Code

Message clean-up

Testplan:
- [ ] Without: Accessing `public.php?service=files&t=` throws an exception
- [ ] With: No exception thrown and 404 page displayed.

Fixes https://github.com/owncloud/core/issues/14231

The TagManager->load() now returns null if the user is not authenticated
instead of failing with an error.

- added existence check for source argument
- removed extra logic for folders, as $view->copy() already supports
  that internally

Cleanup garbage collection for global file cache

otherwise it will lead to failures in json decode and unexpected and hard to debug issues when running the job