Set up live "backup" database on webcompat.com server, updated from suitable webhooks

[hallvors]

Spin off from #165, we are considering having a "backup" database where we store copies of all web-bugs issues when they are created. The idea is to let GitHub send information to our webhooks and use that to update the database. See #780 for code.

The purpose of this database is twofold - one important purpose and one nice-to-have:

1. We can massage the data a little more and for example add a "domain" field to index on - this lets us easily generate RSS feeds and such by domain, see Domain names feed for getting notifications of new issues #60, with less traffic to GitHub.
2. It's nice to have the data around locally, especially if we ever wish to move away from GitHub as a backend.