Promo box widget

[MirandaEcho]

- Choosable background color from site color palette (draws from same filter as Gutenberg color palette)
- Photo can be within box if easier
- Hide image on mobile

[joshdarby]

This widget can probably borrow mostly everything style and functionality wise from #16. I'd think this could be done at around 2 hours (assuming we borrow widget functionality from whatever we create for #16)

[joshdarby]

@MirandaEcho Should the background color selector for this use the same method as #16 where we just replace the default and rev background colors?

Or did we need a selector for every color in the color palette?

[MirandaEcho]

@joshdarby - that works for me, the design only has the blue and green, and this is only used on the homepage.