Technical walk through how feed data updates are detected and loaded in warehouse

[machow]

TODO: go through with @ccjarvus, to ensure the process is sound. Determine path for #235. Currently feeds that are removed from agencies.yml are not marked as deleted in our type2 slowly changing dimension data.

[machow]

Copying in from #277

helpful notes from Chris--there are 4 jobs being done here (should also have 4 dags):

1. put into cloud storage
2. understand files (and when they change)
3. load feeds with changed files into external tables
4. understand what has changed in each table (e.g. build SCD tables)