Add slowness labels based on slowness reports count

What does this MR do and why?

This is very similar to !182 (merged) but for slowness reports.

Thankfully, the heavy-lifting was done in !182 (merged) so the change in this MR is very limited.

I gathered some data (1000 issues) to come up with the slowness:: threshold. I looked at how many issues (Y axis) exists per # of slowness reports (X axis):

# of slowness reports	percentile	Label
1-11	< P75	~"slowness::4"
12-27	P75 < x < P90	~"slowness::3"
28-39	P90 < x < P95	~"slowness::2"
39+	> P95	~"slowness::1"

Closes #62 (closed).