Use fancy underline on install page as well + create helper mixin

[alexandersandberg]

Add helper mixin for underline and use on install page too.

Motivation:

We have a "fancy" underline on the landing page. We should use this on the install page as well for consistency.

Modifications:

• Added a mixin that can be used using @include
• Use it on the landing page
• Use it on the install page

Result:

Before	After