Replacing the body by prepending HTML results in the full content
being re-rendered. This can be a performance issue, but is definitely
annoying when embedding media in ticks.
When a ticker is added by a Gutenberg block, it is initially empty. This
can be detected by checking the "last" flag for 0 value. If found, the
ticker is now updated immediately by the AJAX function and not waiting
for the poll interval to trigger.
