Adds the `sitemap_generator` gem to generate a sitemap.
- Includes static routes and dynamic routes for active crops, plantings, seeds, photos, posts, and members.
- Implements a caching mechanism to ensure the sitemap is generated no more frequently than every 72 hours.
- Updates `robots.txt` to point to the new sitemap.
- Includes a unit test for the caching Rake task.
New route to /robots.txt and some specific logic to switch
on if the subdomain is staging or not.
Staging will not allow any crawlers at all, production does not
have any current limitations
Daniel O'Connor