Evaluate axe-core-gems in feature specs for accessibility testing

Evaluate using axe-core-gems within feature specs for accessibility testing.

TODO

• Add axe-core-rspec gem to Gemfile
• require axe-rspec within spec/spec_helper.rb which extends RSpec with the custom matcher be_axe_clean. Assertions can be modified to use clauses which can be used to modify rules, exclude parts of the page, etc.
• Add expect(page).to be_axe_clean :js within feature specs to view failures related to accessibility.
• Fix tests 🎉
• Add docs