Allow burst PE with bilby_pipe

It would be nice to be able to run on burst events, similarly to how LALInference burst (oLIB) runs.

I've had a little play around with how simple this would be. The good news is that sampling with the burst source model just works.

There following are things we'd have to modify to get this running:

• Make sure the source model in bilby is correct. The current version is https://git.ligo.org/lscsoft/bilby/blob/master/bilby/gw/source.py#L503.
• Make plotting a little more robust, !252 (merged) should be sufficient. Currently, it will fall over because it looks for specifically named parameters.
• Figure out how to make PESummary make a suitable summary page. I guess in the first instance we can just turn off gw mode? This seems to have been independently fixed in pesummary, yay!
• Define burst prior files (online only). See !266 (merged)
• Figure out how to parse cWB triggers from GraceDB (online only). See !266 (merged)

This isn't particularly high-priority, but would be a nice feature to have, and doesn't seem like a huge amount of work. Any thoughts?