No-JS fallback for default color in color picker

[helen]

The no-JS fallback for the color picker is just a plain input. Since the color picker implementation now includes the ability to reset to a specified default, it seems to make sense to include a no-JS fallback for the default, even just as a little bit of hidden descriptive text.

See #22461 for discussion stemming from the custom header screen.

[DrewAPicture]

Patch. Looks like this:

[lessbloat]

+1 to 22509.diff​. Looks good.

[helenyhou]

Thinking we can do it even more generically for color picker in general, not just specific to the screen. Don't have an actual practical solution in mind, just thinking about the possibilities.

[chriscct7]

Its not an ideal solution, because old browsers won't support it, but in HTML5 there is a color input type we could use

[helen]

I don't love the HTML5 color input implementation - I was mostly thinking of a <span class="hide-if-js">#default-value</span> solution wherever we output the field ourselves, including now the customizer control.

[helen]

I've decided this is no longer relevant, and also I clearly spaced when I was thinking about a no-JS thing being inside the customizer :)