Query new ticks using GET request to the REST API.
Also we now trigger one request per ticker, if more than one is
available on the same page.
HTML markup (list element, content container) is generated client side,
s.t. we can work on the generic response data model
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.
