Tweak Announce and Sitevar Banner

Right now there are two ways to notify people of events on the web interface. The sitevar (which keys into classic Emulab) and the announce command (which keys into the portal). I need to remove support for the sitevar from the portal interface so it is announce-only. In addition, I should add options to the announce command line that will modify the sitevar as well as adding an announcement if -p emulab is specified.