Twenty Fifteen: 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.

Add theme support and css classes for gradient background options in the twentyfifteen theme.

I used the theme colors for "from" and lightened or darkened by 10% to get the "to" colors.

Trac ticket: https://core.trac.wordpress.org/ticket/49760

Here's a screenshot of the gradients on the buttons:

[itsamoreh]

I submitted a PR for this. I tried to match the theme's color scheme.

This is my first ticket so let me know if I did something wrong!