Timeline triage

[droberts-ctrlo]

This PR is purely to address the timeline making the system unresponsive. The fix:

• Sets an (arbitrary) limit on the number of items one request can return - in testing on a copy system it was found that this gave us an average wait time of approx 1min when reaching said limit
• Runs the request to update the timeline asynchronously, meaning the system remains responsive, and navigation away from the page is still possible should the timeline be selected by mistake
• Cancels requests should another be required before it has completed

As stated, this is currently a triage operation, this is not expected to be a permanent fix. A full requirement shall be discussed and implemented for v2.7 as far as is feasible.