insights: janky fix for custom time intervals

Created by: coury-clark

I'm a typescript noob so this is a janky fix for https://github.com/sourcegraph/sourcegraph/issues/28874 to get it in 3.35.

Demo gif: https://sourcegraph.slack.com/archives/C01RVEVPWLC/p1639755995263900?thread_ts=1639702730.250000&cid=C01RVEVPWLC