Pinning integration: Build additional Joyride helptext for settings and files screens

[jessicaschilling]

Note: This issue is part of a larger pinning service integration epic undertaken spring/summer 2020.

In brief

Add whatever additional helptext is needed to support pinning service integration in Joyride, as follows:

• Highlight (in Figma doc) all areas that need to be activated
• Write and review helptext in English
• Implement in Joyride, including setting up i18n strings as needed

This issue is dependent upon #1500 and #1501 (stubbing out UI), but can safely be worked once those issues are done.

[jessicaschilling]

Note: once #1615 is merged, I'll start on this.
cc @lidel @rafaelramalho19