Del din liste

You can now share ShoppingLists. 
Made a many to many Migration, which creates the table with keys mapping the many to many relationship between users and ShoppingLists.

Made view changes to show who is on the list.

Seperating SharedLists from PersonalListst.

Made sure if you try to add a user who doesn't exist, it tells the user so.
Made sure if you try to add a user already on the list, it tells the user so.
Made a snackbar telling the user the ShoppingList has been saved.

Userstory: ... I want to share my shoppinglist with others. Done !

For Later: Review how to show which users are currently on the list. If there are many users atm, it will look like poop.