Fix section nav dropdowns on smaller screens

[dereksmart]

Before, section nav was completely broken when you got to smaller viewports. This fixes both instances for viewports above 480px and below 480px.

This also updates the styles to use dashicons to show the arrows.

To test:

• Link this branch to https://github.com/Automattic/jetpack/pull/4339
• Follow the instructions in that PR.

[eliorivero]

lgtm :shipit: