Update pruneThreadIDsSelector to use lastNavigatedTo from threadActivityStore instead of messageStore.threads.
Also as discussed w/ @ashoat, we can check threadMessageInfo.messageIDs.length instead of lastPruned to reduce unnecessarily adding threadIDs to threadIDsToPrune. lastPruned will be removed from threadActivityStore in a subsequent diff.
Depends on D9453
Then why are we keeping styles like border and border-radius?
Don't these styles also affect the <textarea> tag? Are you sure we want to be removing these styles for that as well? Or should we break this into two separate selectors (one for input and another for textarea)?