Use Capybara methods to find/click/check links

We applied the Capybara/SpecificMatcher in commit f52a86b46.  However,
this rule doesn't convert methods finding <a> tags to methods finding
links because <a> tags only count as links when they've got the `href`
attribute. For instance, in the `xss_spec.rb` file we check what happens
when clicking on an anchor tag because we're testing that the `href`
attribute has been removed and so we can't use `click_link`.

So, basically, we can't enable a rule to automatically detect when we're
using `have_css` instead of `have_link`, but we should still do it
because `have_link` adds an extra check which affects accessibility
since it makes sure the tag has the `href` attribute and so it's
recognizable as a link by screen readers.

spec/system/proposals_spec.rb

@@ -27,7 +27,7 @@ describe "Proposals" do
       featured_proposals.each do |featured_proposal|
         within("#featured-proposals") do
           expect(page).to have_content featured_proposal.title
-          expect(page).to have_css("a[href='#{proposal_path(featured_proposal)}']")
+          expect(page).to have_link href: proposal_path(featured_proposal)
         end
       end
 
@@ -36,7 +36,7 @@ describe "Proposals" do
         within("#proposals") do
           expect(page).to have_content proposal.title
           expect(page).to have_content proposal.summary
-          expect(page).to have_css("a[href='#{proposal_path(proposal)}']", text: proposal.title)
+          expect(page).to have_link proposal.title, href: proposal_path(proposal)
         end
       end
     end