Create job backfilling script

[jjnesbitt]

Adapted from @mvandenburgh's initial work.

There was a previous iteration that used the web_hook_logs table. As far as I can tell, that table has been done away with, in favor of a web_hook_logs_daily table, which is somewhat ephemeral. So, we are forced to resort back to constructing the webhook from the underlying db data.

There are other miscellaneous fixes included, which would prevent the backfill script from properly running. These include adding the dotenv job type, and using a different yaml loader (yaml.safe_load is very slow).