Create a membership data model for packages

[tchoutri]

We need to express the membership of a package to a group of other packages, like Core libraries or custom universes of packages (servant, effectful, conduit).

This is particularly important in order to deprecate the @haskell namespace.

We need a table package_groups that contains such package groups, and a many-to-many table whose rows are package_group_id and package_id, that will store the membership.

We need the following actions to be performable from the back-office:

• Add a package to a group
• Remove a package from a group