User can add images, and user has easy-access to create new posts, admin can easily distinguish admin features

- Added admin divs on pages with features accessible only to admins, offset with css, for clarity.
- Added easy-access blocks for creating new ads on posts index and locations index pages.
- Added javascript to build url from dropdown in the location/index page easy-access block
- Added image url field to posts table
- Added image and post body divs in posts show page
- Added css to style new divs in post show page
- Removed comments from gemfile
When there's just one object, don't use an array: `form_for @post`