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
