Add spec_url display/filter support

[sideshowbarker]

For any BCD feature that has a spec_url value: this change adds support for displaying, in the same section of the Web UI where any existing "MDN" link is displayed, a link to the relevant spec.

This change also adds support for filtering feature searches based on the presence of a spec URL.

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Note: It’s (unfortunately) possible for a BCD feature to be associated with multiple specs; so the spec_url value in BCD can be either a string or an array of strings.

Therefore, to handle the possible cases, the patch in this pull request does this:

<% if feature.spec_url %>
  <% if feature.spec_url.start_with?('[') %>
    <span class="card-link">Spec:</span>
    <% specURLs = feature.spec_url.tr('[]""','').split(',') %>
    <% specURLs.each_with_index { |specURL, index| %>
      <%= link_to (index + 1).to_s, specURL.strip, class: "card-link spec-link" %>
    <% } %>
  <% else %>
    <%= link_to "Spec", feature.spec_url, class: "card-link spec-link" %>
  <% end %>
<% end %>

…which isn’t pretty — but the alternative of making the code handle it otherwise is bit beyond my rudimentary Rails and Postgres skills; specifically, I don’t really know how to:

1. deal with reading the spec_url value properly from the BCD data to begin with given fact that the value in BCD can be either a string or an array
2. store the spec_url value in the database with the right datatype (always store it as an array?) or datatypes (sometimes as a string, sometimes as an array?), and
3. read the spec_url value back from the database in the right way

But all that said, if what the existing patch is doing is just too ugly, and if I could get some guidance on how to do it better, I’d be happy to put some more time into refining this.