Handle summaries of vulnerabilities obtained from different sources

[sbs2001]

https://github.com/nexB/vulnerablecode/blob/4933778e4de76f7ea1ffb3fa87c84bd81ee82af8/vulnerabilities/import_runner.py#L236

Currently the order of importers dictate what the summary for a vulnerability should be. This should be fixed by keeping logs, and having a canonical summary.

[sbs2001]

I'm considering https://github.com/jazzband/django-simple-history for logging model changes

[pombredanne]

See also #1393

[DennisClark]

The challenges of handling advisory summaries from different sources has been addressed. Closing.
See #859 .