Set nofollow on links in Markdown content

To disincentivize spamdexing, links in user-generated content should be
disavowed by annotation with `rel="nofollow"` attributes:

  - https://en.wikipedia.org/wiki/Nofollow

Automated spam has already targeted OSEM in the wild:

  - https://github.com/SeaGL/organization/issues/274

Ideally link annotation would be performed during Markdown rendering or
a single sanitization pass, but this is currently an unresolved issue:

  - https://github.com/vmg/redcarpet/issues/720

spec/helpers/format_helper_spec.rb

@@ -24,5 +24,10 @@ describe FormatHelper, type: :helper do
     it 'removes unallowed elements' do
       expect(markdown('<em>*<style>a</style>*</em>', false)).to eq "<p><em><em>a</em></em></p>\n"
     end
+
+    it 'sets nofollow on links' do
+      expect(markdown('[a](https://example.com/)'))
+        .to eq "<p><a href=\"https://example.com/\" rel=\"nofollow\">a</a></p>\n"
+    end
   end
 end