Feature/a11y navigation

[thatbudakguy]

refs #205

• headers don't link to themselves anymore
• "pills" on intervention and footnote cards have title text indicating that they're links
• added some accessibility util css classes borrowed from bootstrap (_a11y.scss)
• added a link visible to screen readers on all pages to skip to the bottom navigation

[codecov-io]

Codecov Report

Merging #209 into develop will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           develop    #209   +/-   ##
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  Coverage     95.6%   95.6%           
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  Files           89      89           
  Lines         4371    4371           
=======================================
  Hits          4179    4179           
  Misses         192     192

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[meg-codes]

This looks good to me from a screen reader stand point (at least, the link works!). 'Skip to navigation' does appear on focus when tab navigating regardless of screen reader, but only when tab navigating, which I believe is the point.