Create manytomany relation on Vendor and VeganDish

Add vegan_dishes ManyToMany field in Vendor. Create schema migration to
generate table based on new relation. Create data migration to populate
new table with exising vendor and vegan dish relations. Remove existing
vendor foreign key from VeganDish as it is no longer needed.
Update with new vendor vegan dish relation

Update forms and models to make use of new vendor_vegan_dishes relation.
Create model admin for VeganDish. Create VendorVeganDish inline and
display on Vendor and VeganDish model admins.