Persist last visited timestamp
Persist and restore unread messages
Fixes new message notifications

Would like to remove this logic out of here.
Updates document title on new message
Fixes first unseen marker
Slight adjustment to day separator position
Adjustments to unseen marker styling
Adds some new styling for unseen marker

Adds is-above-screen and is-below-screen, in preperation for applying
those as needed.
New messages on an active browser, won't be unseen

Probably need to have a more active "brwoser is currently active" thing
going on. Have created #20 to hopefully cover that.
Merge pull request #19 from jeef3/unread-message-improvements

Unread message improvements