fix(Button): support flip for cirulcar buttons

[mintyfloss]

Before submitting a pull request, please make sure the following is done:

I have done all of the following:

• Added a top level class to all my components '.anchor-[COMPONENT NAME]'.
• Used conventional commits for all work.
• Tested my solution on Mobile & Tablet.
• Wrote unit tests for states and all behavior (npm test) and passed coverage thresholds.
• Updated snapshots for all permutations (npm test -- -u).
• Accounted for hover, focus, blur, visited, & error states because they are not edge cases.
• Created TODOs for known edge cases.
• Documented all of my changes (inline & doc site).
• Made sure that all accessibility errors are resolved.
• Added stories with knobs for all possible configurations.
• De-linted and ran prettier (npm run pretty) on my code.
• Added name to OWNERS file for all new components
• If adding a new component, add its export to the rollup config
• package.json version is bumped (if necessary)

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Outline your feature or bug-fix below

My code does...

[mintyfloss]

I have a snapshot to update, but I'm not able to push any changes :(

remote: error: GH006: Protected branch update failed for refs/heads/circular-flip.
remote: error: Required status check "continuous-integration/travis-ci" is expected. At least 1 approving review is required by reviewers with write access.
To github.com:RetailMeNot/anchor.git
 ! [remote rejected] circular-flip -> circular-flip (protected branch hook declined)
error: failed to push some refs to 'git@github.com:RetailMeNot/anchor.git'

[brandonlilly]

if base works below, then circular should work here.

[mintyfloss]

Ah yes, I didn't realize circular was in the theme. It looks like I have more work to do in this PR. Circular flip only works in the md size. The problem is that I want to make the flip be 50% of the button height and make it a square. I haven't thought of a good way to do that yet.

[brandonlilly]

make it a square on a circular button? do you have a mock?

height is something you can pass into the flip component from the button component, then just calc(${height} / 2) should work I think