Extract certificates from votes and make them available on the metrics website

rransom suggested extracting authority certificates while parsing votes and making them available separately. I'll do this in three substeps:

1. Extract certs from archived votes from 2007 until February 2011. Will make them available later today.

2. Extend metrics-db to extract certs while parsing votes, and extend metrics-web to make the certs available on the data page.

3. Extract certs from votes between March 1, 2011 and whenever we deploy 2, and make these certs available, too.