Populate the 'authors' table with unique commit authors.

This migration script finds the unique commit authors from the "master" branch in all of the known repos and inserts them into the "authors" table if they do not already exist (which they shouldn't, but adding a check was easy and allows the script to be run safely more than once). It also finds matching entries in the "users" table (by email) and fills in the "user_id" field.