Add promo box widget

[joshdarby]

Changes

This pull request makes the following changes:

• Adds a new San Francisco Public Press Promo Box widget and styling for it (image uploader portion is stolen from the Largo Image widget)

Desktop:

Mobile:

Reverse background:

Why

For #18

Testing/Questions

Features that this PR affects:

• Homepage promo box widget

Questions that need to be answered before merging:

• [ ] Is this PR targeting the correct branch in this repository?
• [ ] Does the widget image uploader work ok?
• [ ] Did I miss anything that should've been removed while revamping this widgets code?

Steps to test this PR:

1. Add the new San Francisco Public Press Promo Box widget to the homepage bottom widget area
2. Give it a title, text, image, and image link
3. View it on desktop and mobile
4. Switch to the reverse background and make sure it works ok

[benlk]

:shipit: