Attempt to automate the comparison list.

Previously the comparison dropdown list was managed manually, since we have the features.yml file now we should be able to automate it.

This doesn't currently work properly, opening this so I can get help from someone else.

What it looks like with this MR:

What I want it to look like:

Page is here: https://about.gitlab.com/comparison/

This is what I'm trying to do (it's not in the current MR because it causes the page to throw an error), but data.features.comparisons isn't coming out properly, not entirely sure why:

= data.features.comparisons.each do |comparison|
  - if (comparison.link != nil)
    %li
      = link_to(comparison.title, comparison.link)