Added publishing functionality to Pubsubhubbub

Added the functionality of sending the alert to Pubsubhubbub.

Right now we have all the valid info only about pubsubhubbub . Will add a broker table later as and when more brokers are available.
This broker list is temporary -- the next step is to come up with a real way to organize brokers.  (Preview:  There are several issues:  a) In a multi-tenant site, each org may have its own list of brokers.  b) Pubsub brokers are gateways -- the "real" way to use them is to have a routing table that says, for each subscriber (recipient in the alert's recipient list), which gateway to hand the message off to.  We don't have a scheme for doing that yet.  And we might decide that's the wrong model -- that this isn't really pubsub, but rather (say) we send to brokers based on properties of the alert itself (e.g. language, type of incident,...).)  However, we could add the multiselect now, if we think we're going to need it anyway.  If the brokers end up in a table, we could populate it from the table instead.