Update Project Table constraint on entity_id and entity_type

Currently an entity can have only one child Project because of a unique constraint on entity type and entity ID. We want Projects, CiviEvents, and other entities to be able to have multiple children.

It is impossible for there to be two records for the same project, therefore, there is no need to protect against a project having multiple parents.

The constraint needs to be removed both for fresh installs and for upgrades.