store time-to-X metrics for trend graphs

[GregSutcliffe]

felixfontein

seeing it over a few months would be helpful I think
for detecting orphaned repos, it might be useful to even have it over years, to see how it used to be in the past :)

[GregSutcliffe]

There is some completion on this, we store time-to-merge and time-to-first-comment trends now. Although I think there is a bug in the display, sometimes issues & prs are swapped - currently community.general displays this:

Clearly, issues trend should not be better than prs trend