Make notification times visible in drop-down

[cellio]

meta:294223

The notifications drop-down shows relative times (1 day ago, etc). Usually when we show relative times, the actual times are available on hover -- but that's not the case here. I also noticed that those times flash briefly before being replaced by the relative times, so the notification dropdown has the information.

Can we show the timestamp on hover like we do elsewhere?

[cellio]

Related, bigger: #946