Threading still doesn't work for some messages. To debug this more
easily, especially for non devs, this adds a new action menu entry to
download all relevant threading data as a simple JSON. The maintainers
can then look at those files, put them into unit tests and see why they
messages don't group.

Again, this entry won't be shown on production instances but only if
debug mode is enabled.

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>