Flow implementation for secure view

[juliusknorr]

In order to properly handle VDR requirements this PR implements a Flow operation in order to enable secure view through the workflow engine. File access is then controlled by a storage wrapper similar to the one in files_accesscontrol in order to make sure that the file content can only be obtained when opening documents in Collabora where watermarking and downloading can be controlled.

• Properly register operation through an event listener
• Implement proper checks for the collabora remote
• Avoid fopen on /index Emit read hook instead of fopen for admin_audit #1957
• Possible enhancement: Add options to operation to show different watermark or fine tune the secure view options
• Figure out how should handle unsupported filetypes (still block or leave them up to the admin if flow is not limited to mimetypes)

[juliusknorr]

Aiming for a different approach now with #2268