Tell the user which messages arrived since they last saw the browser tab.

When the user switches context away from our browser tab, draw a line under
the last seen message, so when they return they know which messages are new.
Merge branch 'development' of https://github.com/ehues/KiwiIRC into ehues-development

Conflicts:
	client/src/views/application.js
Keeping the .last_seen message marker in place until window blurs again