Lea add locations prices to posts

Added a dropdown to the new post form, from which users can choose their location.
Added a field for entering the posted item's price to the new post form.
Wrapped the new post form in a signed-in? conditional, so only signed-in users see the form.
Use `location` instead of `location_id`. Then it will check to make sure the location actually exists (instead of just having a value in the `location_id` column, and it will have a better error message. I.e. "Location is not present" instead of "Location id is not present".