[WEB-408] Enable header image for student groups

[nathanbillis]

On the member dashboard there is an option for header image, we should enable it for members to use.

For Pluto, calling the /groups and /groups/[id] endpoint should reveal the new header_image_url variable as part of the response ready to use.

_WEB-408

[linear]

WEB-399 Enable header image for student groups