wui: improved columns in welcome page server list

As discussed at https://tahoe-lafs.org/trac/tahoe-lafs/ticket/1973

 - replace lengthy timestamps with human-readable deltas (eg 1h2m3s)
 - replace "Since" column with "Status" (addresses usability problem in #1961)
 - replace "announced" column with "Last RX" column
 - remove service column (it always said the same thing, "storage")

This is a squashed commit of 3fe9053134b2429904f673df561e602a50f83c7e
Thanks to an anonymous contributor who wrote some of the tests.