Create scraper to update legislator information

[aih]

See https://github.com/unitedstates/congress-legislators/

https://github.com/unitedstates/congress-legislators/blob/master/legislators-current.yaml
(Current yaml file of legislators)

Ideally, we will include this information in the cosponsors table, and get that information (with the bioguide id) when populating the context for the bill-detail page.

This issue is to create a scraper + Celery task to:
a. download the legislators and committees yaml files
b. process the files and update the sponsors database

We will create a separate issue to associate the information with the sponsors in the table.

[aih]

Working in common/cosponsor.py