Let's switch to h2 for sub headings

[dnmfarrell]

In our articles we use h3 as subheaders:

### sub header

Let's switch to H2. It makes more logical sense (if the title is H1), but it will also let us write articles with the nicer underlined sub headings if we want:

sub header
----------

One way to do this would be:

1. Check if anything is using h2 on the website at the moment, if it is switch it to an alternative
2. Update our css to style H2 and H3 in exactly the same way, check that they look the same
3. s/h3/h2/ for all articles.
4. style H3 like H4 is currently
5. s/h4/h3/ for all articles

[briandfoy]

If you set up the CSS I'll go through the articles to convert things.

[dnmfarrell]

Ok @briandfoy this is live. This example article uses H2 subheadings. Commit 334901a