Move endpoints for lists into their own file

[hepplerj]

Right now for the Bills of Mortality, we have three lists of data that are used for populating the UI and filtering data as users select them:

1. Parishes (distinct)
2. Causes of death (distinct)
3. Christening descriptions (distinct)

Parishes has its own endpoint, but Causes and Christenings exist within their respective files (bom-causes.go and bom-christenings.go). Just to make this a bit easier for our mental model of how the endpoint works, I may move these into a new b om-lists.go file that accepts query parameters that returns the specific list. Instead of /bom/parishes returning a list of parishes, we might have something like /bom/lists?type=parishes and so forth.