Issue description

Multiple users should be able to edit a task at the same time
If one user edits a segment, this segment is locked for other users
If one user saves a segment, this segment get immediately updated for other users (including comments)
repeated segments: repeated segments are also locked
if there are multiple users on a task the replace all functionality is disabled automatically

Possible Implementation via Web Sockets

Excel Reimport as Worker: Currently the excel reimport is running in a direct (blocking or synchronous) worker. The reason is just to be able to print errors to the user coming from the reimport. This is not possible with the worker running in the background. With websockets indeed it would be possible.
Remove the pull loop in the task overview which pulls tasks in a locked state and replace it with websockets

1.	Optimize application server connection	Backlog	Thomas Lauria
2.	Make instance ID more flexible	Backlog	Thomas Lauria
3.	Secure websocket connections against origin domain	Backlog	Thomas Lauria