Ensure `erc-gitter-notifications` is required.
Implement overlays for unread notifications.
Fix notification and overlay clearing to properly reflect expected
behavior.

Switching to `*Gitter Notifications*` buffer will clear pending count
but not move overlay.  Clearing notifications will clear pending count
and move overlay to `(point-max) (point-max)`.
Improve logic on unread overlays.

Do not make the overlay variable local, this would result in the
variable being moved incorrectly.

Change buffer creation logic to avoid additional overlay creation.
Remove notification pending notice when moving between notices (if
buffer is already visible).