Migrate the database schema to SQLAlchemy

The new schema was generated with sqlacodegen and then manually adjusted
to fit schema/aur-schema.sql faithfully, both in the organisation of the
code and in the SQL generated by SQLAlchemy.

Initializing the database now requires the new tool aurweb.initdb.
References to aur-schema.sql have been updated and the old schema
dropped.

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>