Add country data to drawings

[steverabino]

To address https://github.com/empowerhack/DrawMyLife-Service/issues/31

Adds string location to Users and Drawings

For Users, this will be required on sign-up (or next profile edit), and will show Greece, Hungary, and Serbia are preferential countries (though all are shown)

For Drawings, if a new drawing is being created, it will default to the current user's country, but can be changed when entering details for the picture

If an existing drawing is being edited (that has no country data), it will default to nil ("Country picture was drawn in"), as it could lead to accidentally updating the pic with the wrong data.

• [x] Remove model for locations and references to model
• [x] Add country gem
• [x] Add country to User model (migration & validation)
• [x] Add country to New/Edit User views
• [x] Add country to Drawing Model (migration)
• [x] Update Drawings controller to use country instead of location id
• [x] Add country to New/Edit Drawing views
• [x] Add country to Show Drawing view
• [x] Default country to User's country on New Drawing view

[steverabino]

Known Issue: There are a few instances of priority countries for dropdowns. These are hardcoded so far to Greece, Hungary, and Serbia.

Once we have an app-wide settings view, this can be integrated into that.

[steverabino]

@krissy - all good for review 🙈

[krissy]

Awesome work! 💃 Just a couple of little comments but overall :+1: :D