Move article styles to mixin
Move article styles to mixin

Summary:
This will allow us to only specify the variable properties when implementing tablet and phone
styles.

Test Plan: Same as D27590.

Reviewers: david, kevinb

Reviewed By: kevinb

Subscribers: jared, benkomalo

Differential Revision: https://phabricator.khanacademy.org/D27592

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