Move Bullhorns to use Server Sent Events, rather than the current polling approach

Bullhorns currently uses a polling approach, on a schedule controlled by a Sakai property. It will be a lot more efficient connection wise to use server sent events and SSE will also make bullhorns more responsive. You will see realtime updates to the alerts. As part of this work I'll make this approach reusable across all of Sakai. I'm thinking of MOTD stuff, popups with critical dates like imminently due assignments,  hot indicators on tools to show activity. Triggering quiz overlay popups in the browser. SSE will make all that stuff a lot more feasible.