Add column to indicate whether a cosponsor is no longer in Congress

[aih]

Related to #318

From discussion today: We will consider adding a column showing whether a cosponsor is a current member of Congress; the Cosponsors db table contains only current members, so if the bioguide id of the cosponsor is not in the Cosponsors table, the cosponsor is not currently in Congress.

Assigning @nkinaba and @smplater to add any prior mock-up we may have had for this feature and @DanielSchuman to suggest what the column should show (e.g. Yes/No for current membership, or only a No for people who are not currently members?)

[nkinaba]

[aih]

We do not show cosponsors who are not in currently in Congress in the 'Sponsors of Related Measures' table; we get that data from the Bill-Cosponsors table, which only has current members.

So we can add the 'Currently in Congress' column to the Committees and Sponsors table in the top right.

[aih]