Re-open closed mentions.

The new Mentions / Unreads panel is great! But it lacks the ability to re-open a closed mention - even if it's done by accident. I'd suggest adding a hidden tab of "Closed Mentions" where these unwanted mentions are dismissed to. Or, alternatively, if people would prefer that closing the mention gets rid of it altogether, I'd recommend a short period of time where an "UNDO" toast message / modal is available, so the action can be reverted in case a user does it by accident, maybe 3 - 5 seconds or so. 

Thanks!