Show live relative timestamps in the datagrids.

The "X minutes ago" timestamps in the datagrids were still being
computed on page load. Now they're live, just like the rest of the UI.

Watched the page for a bit after making changes. Saw that the timestamps
were updating.

There is a bug, which is that if you add a column for relative timestamps,
they won't be shown until the next dashboard reload. This is due to not
knowing to call timesince() for new columns. I'll be working on a fix for
this separately, as it'll require a little bit of extra work. For now,
it'll just show the date until it reloads.