more dashboard CTA adjustments

[gumaerc]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/9194

Description (What does it do?)

This PR makes some style updates to the CTA buttons in the Learn dashboard, described in the issue above.

Screenshots (if appropriate):

How can this be tested?

• Make sure you have an instance of MITx Online up and running and connected to the same Keycloak / APISIX instance as Learn as described in the README
• In your instance of MITx Online, make sure you have a B2B organization with a contract and some courses added to said contract, and that your user is part of said organization
• Visit your org dashboard in Learn and verify that the logic surrounding the CTA text / icon reflects what's described in the issue above:
  • Courses that haven't been started show "Start Course" or "Start Module" with no arrow
  • Courses that have been started say "Continue" and have a right facing arrow
  • Courses that are completed or expired show "View Course" or "View Module" with no arrow

[ChristopherChudzicki]

👍