Remove the 'Sponsor' model from bill.models

[aih]

The 'sponsor' model has no data, either online or on my local instance. It seems that the model is a leftover from earlier development and design. The Cosponsors table does have data, and roughly corresponds to the Cosponsor table on Govtrack:

https://github.com/govtrack/govtrack.us-web/blob/master/bill/models.py#L107

We should remove 'Sponsor' wherever it is found in this repo. Separately, we should update the Cosponsor model to be more similar to the one on Govtrack, which includes a join table and more information about a cosponsor's role in the bill.

[aih]

Done