Added project limit logic

Users should have 1 project per week. They should never have more than
one.

This commit adds the `projects_available` field to the DB, defaulting
to 1.

It also adds the logic for decrementing when the user creates a
project, incrementing (but not past 1) when the user deletes a project,
and displaying the “Create” link conditionally depending on how many
projects the user has.