Ensure that themes support sitemaps by default to automatically work with Search.gov

[eddietejeda]

User Story

As a Federalist that would like to use Search.gov, I need to install jekyll-sitemap to keep my indexes up to date. Instead of requiring users to install this themselves, jekyll-sitemap could be provided as part of the standard package.

Background

More information: https://help.github.com/articles/sitemaps-for-github-pages/ cc: @konklone @karentrebon

Acceptance Criteria

• [ ] Evaluate whether adding support for jekyll-sitemap by default is backward compatible.
• [ ] Add jekyll-sitemap to the default Jekyll themes
• [ ] Release a new version of default themes.

[afeijoo]

We're also running into this issue on several Federalist sites that use Search.gov.

cc: @dawnpm & @wslack