Supply naming_convention to SQLAlchemy metadata

[jace]

Since 0.9.2, SQLAlchemy has supported an explicit naming convention for foreign keys, indexes and constraints. Alembic also supports reading and honouring this naming convention.

Coaster should supply a naming convention that matches our conventions so far (and not the ones recommended in the Alembic manual) to ensure our migrations don't break in future on new database deployments.

[jace]

As a counterpoint, we should adopt the recommended conventions and write migrations across all apps to rename the existing constraints.