#1304 Allow searching secondary verifications

[Ahasuerus]

Allow searching secondary verifications. This will help find verified pubs which fall outside of the valid date range of the verification source. The UI should match other Advanced Search UIs. Supported selection criteria should include verifier, verification source, and year. Operators should include "before" and "after" for years.

[Ahasuerus]

Part 1 - Added Secondary Verification Source as a selection criterion to the Advanced Publication Search page:

biblio/advSearchClass.py
biblio/adv_search_results.py
css/adv_search.js

Installed in SVN 475 on 2019-10-31. Keeping the FR open to see if we still need to create a separate Advanced Secondary Verification Search option.