Make the hero image overlay gradient a boolean, instead of hard-coded

[law]

The hero-image overlay gradient here, specifically 'bg-gradient-to-r', looks great, but it doesn't fit with the hero image I'd like to use. Are there any strong opinions around making it activate based on a site-wide boolean that is by default set to 'true' (so it doesn't mess with backwards-compatibility)? I have this working in a local branch I can PR for further testing if there is interest.

[law]

I took the liberty of forking and PR'ing a change here so folks can more easily see what I'm talking about - #1063