Use "AUTOINCREMENT" for user table primary key

[yzernik]

I noticed in my app that after I delete a user, when I create a new user, the new user has the same id as the deleted user.

This behavior is not expected. It can be avoided if this line is changed to include AUTOINCREMENT: https://github.com/tvallotton/rocket_auth/blob/ef445dfe22c6738e95c21f5710f85ab1136bdb06/src/db/sqlite/sql.rs#L3