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!
I have done a ton of stuff on this pr, but I'm super tired and don't wanna work on this damn test.  

Can you please update it so it does the following:

* Creates a user
* Creates a opp instance with an association to that user (see admin save)
* Makes sure that you can retrieve the model by that user (see admin list)
* Destroys the instance 
* Destroys the user.

Thanks @chrisrodz