properly add Mit-AdornmentButton class

[gumaerc]

What are the relevant tickets?

Follow-up for https://github.com/mitodl/hq/issues/5386

Description (What does it do?)

This PR addresses an issue with #1545 where a class was used that was not properly defined addressing adornment buttons.

Screenshots (if appropriate):

Good:

Bad:

How can this be tested?

• Spin up this branch of mit-learn
• Visit the home page at http://localhost:8062/
• Ensure that the search button is the proper width
• Visit the search page at http://localhost:8062/search and verify the same thing
• Visit the unit search page at http://localhost:8062/c/unit/ocw/ and verify the same thing

[ChristopherChudzicki]

👍

[ChristopherChudzicki]

Merging for Carey, he's on vacation.