Split the save keyboard shortcut into save and saveRES

Suggested in #417.

This does change existing behavior (the save key now saves comments to reddit instead of saving them to RES, which now has its own key), but I think most users who are smart enough to save posts to RES should be able to figure out the new shortcut.

This also allows the save-RES status to be toggled before a refresh (i.e. you click `save-RES` and it becomes `unsave-RES` and can be toggled, but after a refresh it becomes `saved-RES` and you must go to the saved page to delete it), which I think is a fair balance: users can easily unsave something if it was an accident, but can't accidentally unsave something that they are revisiting.
Removing `.empty()` is the cleanest way to keep displaying my text when there are no saved comments.
It didn't seem to do anything regardless, since no children are added to savedCommentsContent when there are no saved comments (perhaps I'm overlooking something?), but removing it didn't seem to cause any ill effects.