appview/timeline: collapse identical events

Group consecutive timeline events (defined as being the same action and operating on the same target) together so that the same target is not seen multiple times in a row. This notably does not completely aggregate all events together, so the timeline may still appear as "A B A" if several instances of event A were broken up by event B. For collapsed groups, we render "and N other user(s)" in the description of the event, with a popover that shows the collapsed users if "N other user(s)" is hovered. The number of events fetched was increased so that `limit` refers to the number of items in the timeline post-grouping. Future follow-up work for this PR could include doing some less naive aggregation of events so that only a single instance of each event would appear on the timeline to further improve the diversity of what gets shown; this PR was intentionally kept minimal.