Support sitemap generation

:fork_and_knife: 

Adding support for sitemap generation as part of https://trello.com/c/e68KlAKz

I added the [sitemap_generator](https://github.com/kjvarga/sitemap_generator) gem to the project and have created a Sidetiq worker that runs daily at midnight which refreshes the sitemap.

What gets added to the sitemap:

- the welcome page (root or `/`)
- the cookbooks directory (`/cookbooks/directory`)
- individual cookbooks (with a higher priority than cookbook versions)
- individual cookbook versions
- individual users
- ICLA signatures path (contribute or `/icla-signatures`)
- CCLA signatures path (contribute or `/ccla-signatures`)