#3027421 Plans: Update descriptions and pricing styling for Jetpack

Changes proposed in this Pull Request

• Updates Jetpack plans tables according to the latest pattern in Calypso; adds audience highlight.
• Updates pricing section in signup plans tables.

Before, Calypso > Plans

After, Calypso > Plans

Before, JP Connect > Store

After, JP Connect > Store

Testing instructions

• Fire up this PR.
• Visit http://calypso.localhost:3000/plans/SITE_URL and ensure the audience highlight is visible (on both mobile and desktop).
• Visit http://calypso.localhost:3000/jetpack/connect/store/ and ensure the pricing section looks good on desktop (mobile is unchanged).