Fix regressions for the ROI calculator

What does this MR do?

• Changed the color of the even rows for the features table to a lighter gray
• Fixes some inconsistencies with the mobile design
• Adds a (USD) suffix to the label Average employee cost per hour as to specify the currency used for the savings

Screenshots

Lower contrast on the features table

Mobile version

Which issues does this MR help solve?

https://gitlab.com/gitlab-com/www-gitlab-com/issues/1485