Twenty Nineteen: Add gradient background options that fit the theme color scheme

[ianbelanger]

Now that we have the option for certain blocks to have gradient backgrounds, we need to add gradient background options to the theme that fit the themes color scheme.

I've added support for block gradient presets in the Twenty-Nineteen theme. It currently grabs the primary color and sets a linear gradient to 80% of the color.

Trac ticket: https://core.trac.wordpress.org/ticket/49712

[lamdayap]

Hi @ianbelanger, this is one of my first handful of contributions to any sort of project and I'm very excited to try my hand at this issue! I've added a single preset to the Twenty Nineteen theme, it gets the primary color and sets a gradient to 80% opacity. I've adjusted this to be reflected in the color-patterns.php file as well. Would love to hear your thoughts. It's possible I may have missed a few places where I also need to edit the code, I'll continue to work on it. :)