Fix XOM styling of explanation link
Fix XOM styling of explanation link

Summary:
While the styling of the explanation link for XOM looked correct inside the
perseus demo, the color and text-decoration was being overwritten by the Link
component that was being injected in to perseus from webapp.

Instead of trying to override those styles which turned out to be convoluted,
requiring changes in both perseus and webapp, I've opted for a simpler solution.
This diff renders an anchor tag and applies the XOM styling to that element.

Test Plan:
- build within webapp
- load http://localhost:8080/math/early-math/cc-early-math-add-sub-100/cc-early-math-add-ones-tens/a/adding-1s-and-10s-practice
- verify that XOM styles appear in mobile emulation mode in the browser
- verify that original styles appear in the desktop browser

Reviewers: charlie

Reviewed By: charlie

Subscribers: jared

Differential Revision: https://phabricator.khanacademy.org/D29730

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