Create many-to-many relationship between packages and external users

[tchoutri]

We need to account for external users when dealing with Hackage indexes. We need a table for external_users with a couple of preliminary information:

• id : surrogate key that we manage
• username : username from the remote Hackage index
• external_id id of the username in the remote Hackage index
• package_index foreign key to package_indexes

Then we need a many-to-many relationship package_maintainers that links packages and external_users