Updates for WCAG2AA compliance

[janette]

fixes #3079

• Update data-format colors for contrast
• Add scope attribute to Additional Info table on dataset page

QA steps

contrast

• Add dummy resources to a dataset (title and format only) for
  • html
  • geojson
  • xml
  • text
  • xls
  • data
• view the dataset with the search page search/type/dataset
• run WCAG2AA test (drag bookmarklet to bookmarks bar https://squizlabs.github.io/HTML_CodeSniffer/)
• confirm you get 0 errors

table cells

• go to a dataset and run the WCAG2AA test
• confirm you do not get errors on the Additional Info table

[dafeder]

Got an error on the search form at the top of /search

[dafeder]

Errors seem to be particular to Firefox on Linux, or maybe specifically my Firefox (don't appear for me in Chrome, don't appear in Firefox for mac)