Increase font size for H3 elements
Increase font size for H3 elements

Summary:
The new XOM typography styles will enforce better differentiation for headers (vis-a-vis body
text), but in the near term, I've applied some of the styles to all articles.

On Asana @ https://app.asana.com/0/20971210297637/134792002514536/.

Test Plan:
- Patch in the change.
- Open up an article.
    - Verify that the H2s are larger than before, but smaller than the H1s.
    - Verify that the H3s are larger than before and bolded, but smaller than the H2s.

Reviewers: kevinb, benkomalo

Reviewed By: benkomalo

Subscribers: jared

Differential Revision: https://phabricator.khanacademy.org/D27728

ariabuckles/perseus,ariabuckles/perseus,learningequality/perseus,learningequality/perseus,ariabuckles/perseus,ariabuckles/perseus