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.
This is how you do it:

```
form.collection_select :location_id, @locations, :id, :location
```

i.e. drop the first argument (`:post`), because you get that for free when you do `form.collection_select` instead of the plain `collection_select`.