Observed behavior (please include a screenshot if possible)

In some browsers (Firefox?), when the user closes the tab/window or navigates away from datatools, componentWillUnmount will not necessarily be triggered, and changes to feed source labels in the "General Settings" tab may be lost. The correct DOM event to handle is sendBeacon, which is currently not compatible with datatools server (only unauthenticated POST requests are allowed when sendBeacon is triggered).

Expected behavior

Changes to labels should be persisted.

Steps to reproduce the problem

1. Set up labels in a project.
2. Open a feed source and go to the "General Settings" tab.
3. Tick one or more labels
4. Close the browser tab
5. Reopen the feed source general settings in another tab
6. Notice the changes to labels were not persisted.

Any special notes on configuration used

n/a

Version of datatools-ui and datatools-server if applicable (exact commit hash or branch name)

Current versions of datatools (both UI and server)