Localized language switcher in footer

I proposed this change in issue #336. This change should allow users
to see available translations in their own language. This change also
allows for alternative UI implementations of the language switcher.

Summary Changes:

- Added section to the locale called 'localelinks'
  - intro: Localized content equivalent to the English "Content available in "
  - separator: Localized content equivalent to the English ", "
  - conjunction: Localized content equivalent to the English " and "
  - full_stop: Localized content equivalent to the English "."
  - text: Localization of the language name, eg. "English" or "Español"
  - url: The url to switch the language displayed
- Updated footer include so locale links are not hard-coded to English

Known Issues:

Non-English translations in _confg.yml site.locales.localelinks are
based on Google Translate.  These should all be verified by native
speakers.