Drop GitHub tables before each run

[ghickman]

Now that we're backfilling all data on each run we can simplify some of that by dropping the table each time we run the backfill. We still maintain the upsert functionality so this is not entirely necessary but it also helps in local development.

The backfill time is down to <2m locally and the expectation is that we'll run backfills in the middle of the night so I'm not expecting this to cause problems for users in production.

While we only have one GitHub table currently this looks for all github_* tables on the basis that we're not ingesting all the data all at once so future tables (eg issues) won't have to worry about doing this too.

[benbc]

Doing a proper review later post-merge.