Twenty Fourteen: 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 gradient background options (Twenty Fourteen)
Trac ticket: https://core.trac.wordpress.org/ticket/49761

[adeltahri]

I submitted a PR for the ticket.

[karlijnbk]

Hi! I've tested the PR and the colours look really good, but I might have found one possible problem.

On trunk, when you change the button color to a gradient option, it immediately adds a gradient to the button color. On the PR however, the color is solid and only changes to gradient when I, for example, change the angle.

See the two recordings below for clarification.
Trunk: ​https://www.loom.com/share/5a4a54d73aad4137a94fcd1c3f4c4fdf
PR: ​https://www.loom.com/share/3875577390de409aa909e0e9d262fd2d

I'm also adding some screenshots to show the beautiful new gradient colors.

[karlijnbk]

Theme colors and new theme gradient colors

[karlijnbk]

Gradient colors on buttons