Project creator may set a project header image

[zspencer]

In order to build trust and rapport with potential supporters
As a project creator
I want a way to be able to express my project visually with a header image

Scenario: Uploading a header image
Given I have created a project
When I upload a header image
Then the header image is visible on the project page

See https://docs.google.com/presentation/d/1Gv5yrsLC1qSQJi5njoFPGGZjcVAbox_SYu-hCFYcLs0/edit#slide=id.g2f83648169_0_0