Jumbotron configuration upgrades

[beckymargraf]

• Ability to set jumbotron_image behavior as cover/scale-to-fit or tile
• Ability to manually customize fill with hex color input (rather than a tinted transparent image)

[dirkcuys]

My preference for course in a box is to simplify, ie. only provide 2 ways to configure the header:

A color OR a custom image

More options are possible, but goes into the realm of "here is how you can update the CSS to customize your header further ..."

[beckymargraf]

Okay!

Should we remove the support for the mask in general and just encourage legibility consideration to whomever is designing the course and picking an image/color?

[GrifP]

Related to this: if you remove the default image in the config file and don't replace it with anything else, it will still use the pattern background (rather than the expected result, which would be to not use any image). The workaround for now is to delete the pattern svg from the img folder and then ignore the error message you get when pushing changes, but that isn't intuititive.

Relatedly -- Jekyll needs to be relaunched for config changes to appear. So when we make updates we should reflect that in the documentation.