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.
The .merge should be up in the create method.
We can pull in the current_user_id in the create method and then merge it into the post_params