Distinguish point events from ongoing events in metrics timeline

The metrics timeline does not distinguish entries that represent a single point in time from those that started but have not finished yet. They both are represented by a non-empty start date column and an empty end date column. For example (some columns omitted):

=start date =	=end date =	=description =
2017-10-05		geoip database updated.
2017-10-07		Increase of users in Romania.

We need to disambiguate these two cases for the sake of uses like legacy/trac#24260 (moved), which wants to present date ranges as text.