wui: improved columns in welcome page server list

wui: improved columns in welcome page server list

As discussed at https://tahoe-lafs.org/trac/tahoe-lafs/ticket/1973 and in
previous pull request #129.

 - replace lengthy timestamps with human-readable deltas (eg 1h 2m 3s)
 - replace "announced" column with "Last RX" column
 - remove service column (it always said the same thing, "storage")
 - fix colspan on 'You are not presently connected' message

Previous versions, some with github comments: https://github.com/leif/tahoe-lafs/commit/3fe9053134b2429904f673df561e602a50f83c7e , https://github.com/leif/tahoe-lafs/commit/486dbfc7bd3c0bbba42a6df8e4564601120aec0e , and https://github.com/tahoe-lafs/tahoe-lafs/commit/c89ea625803be36a18bce1af4eef95dcd78bba2b

Thanks to an anonymous contributor who wrote some of the tests.