Enhance /v2/actions to allow negative page numbers.

Often, we want "the last ten actions". The cleanest way to do this would allow a call like, `/v2/actions/...&per_page=10&page=-1`. If we had 192 actions this would return actions 183 to 192.

The current kludge is to ask for the first action, get the number of actions from meta, play a numeric game to get the last ten or more actions on one page, then ask for that page, then filter out some items.