Add AlertSourceType model

[ddabble]

AlertSource.type is currently a "choice field" with hardcoded choices. We should convert this into a foreign key to a new model - AlertSourceType, which enables users to register their own alert source types. This should be done in conjunction with adding glue services for incoming alerts (see #44), instead of having code in the backend for every alert source type, as is currently facilitated for (see mapping.py).