Associate users and opportunities

### User - Opportunity Association

@MrMaksimize please review.

Note that the association is completely application-based with no actual restrictions on the database. Sequelize's fault. Anyways there's a test for this as well, so it works!
The listAll is for the future when we want to have a page that displays ALL opportunities, regardless of if they belong to a user or not.

You can comment it out for now, I just wanted to have it there already.