Don't hide announcements after the last hidden one.
Don't hide announcements after the last hidden one.

Previously, the "hide all announcements before N" cookie was setting N to "the
time the user clicked hide".  Now it's setting N to "the creation date of the
announcement the user is hiding".

For https://unisubs.sifterapp.com/projects/12298/issues/700003/comments

ReachingOut/unisubs,pculture/unisubs,eloquence/unisubs,eloquence/unisubs,norayr/unisubs,eloquence/unisubs,ofer43211/unisubs,wevoice/wesub,wevoice/wesub,ReachingOut/unisubs,pculture/unisubs,norayr/unisubs,pculture/unisubs,ujdhesa/unisubs,ujdhesa/unisubs,pculture/unisubs,ofer43211/unisubs,norayr/unisubs,wevoice/wesub,eloquence/unisubs,ujdhesa/unisubs,ofer43211/unisubs,ReachingOut/unisubs,norayr/unisubs,ReachingOut/unisubs,ujdhesa/unisubs,ofer43211/unisubs,wevoice/wesub