Allow more than one vegan dish of the same name

The migration just removes the constraint on VeganDish.name, should be a fairly lightweight change.

Some things to consider:
We should look into what happens when a vegan dish is removed from a vendor or assigned to another vendor (<-- this probably shouldn't be allowed). 
Removing a dish from a vendor is a possible case, as restaurants can change their menus.

Addresses issue #103