Listen to GCN Fermi GBM_ALERT channel for earlier warning of Fermi GRBs; fixes #396

This MR lets us listen to the earliest alerts Fermi sends out (usually 15-20 sec before FLIGHT notices). Although these tend to be much lower in confidence, we won't be letting these trigger the RAVEN alert pipeline by always applied the NOT_GRB label (note that follow up notices can remove this label and clarify the confidence of an event).

We may want to add similar earlier warning notice types for Swift and other experiments as well.