timelines.js: StatusesSummary: implement showSummaryOf() method. Pass the test.
timeline.tt: add StatusesSummary.
timeline.tt: show warning message when it tries to load too many unacked statuses.
timeline.js: StatusContainer: increase AJAX timeout for POST ack.json, because ack can be a very long process when there are many unacked statuses.
timeline.js, timeline.tt: StatusesSummary: improve view of accordion summary entries.