797472 code sample models

[lmorchard]

Do merge this, if it seems good.

Going to try something different here: This PR consists of a series of commits, each a discrete change that should almost stand on its own and not break the site. Altogether, the PR implements most of a feature, but there are a few things left to wrap up.

What you should be able to spot-check on a dev VM, though, is this:

• Add some code listings inside a container div (in source view, because there's no good editor yet), something like this:

  <ul id="s2">
      <li><span>HTML</span>
          <pre class="brush: html">
              &lt;div class="foo"&gt;
                  &lt;p&gt;Hello world!&lt;/p&gt;
                  &lt;p&gt;Unicode fun: Przykłady 例 예제 示例&lt;/p&gt;
              &lt;/div&gt;
          </pre>
      </li>
      <li><span>CSS</span>
          <pre class="brush: css">
              .foo p { color: red; }
          </pre>
      </li>
      <li><span>JS</span>
          <pre class="brush: js">
              window.alert("Hi there!");
          </pre>
      </li>
  </ul>
  

• Take the URL of the page, add $samples/sample2, eg:

  https://developer-local.allizom.org/en-US/docs/User:lmorchard/TestCodeSamples$samples/s2

• Upon visiting that URL, you should see the results of the code sample as a live page

[jsocol]

jinja2.escape is probably faster unless you need some behavior from sax.

[groovecoder]

test and spot-check pass.

rebase, pep8 cleanup and jinja2.escape change (if you want to)

r+

[lmorchard]

Rebased and did some code cleanup for pep8, but didn't address all the warnings. There's a lot to be done there, unrelated to this PR