Add support and docs for color input

[mdo]

As shown in Safari:

• Add new .form-control-color modifier class to limit the width
• Add basic code example
• Document styling details, browser support, etc

/cc @twbs/css-review

Preview: https://deploy-preview-29059--twbs-bootstrap.netlify.com/docs/4.3/forms/form-control/#color

[XhmikosR]

Perhaps we shouldn't document browser support? I mean, we'd need to cover macOS, Linux and Windows for each of the browsers.

[MartijnCuppens]

I 've added rounded corners to the swatches and fixed the paddings in Firefox. I've also added the a fix which removes padding from webkit browsers in reboot.

[XhmikosR]

Should we add cursor pointer?

Or a title or something. I feel like it's not clear one can interact with it.

[MartijnCuppens]

Yeah, that might be a good idea. Shall we add it with the $enable-pointer-cursor-for-buttons variable?

bootstrap/scss/_buttons.scss

Line 14 in dd383ff

cursor: if($enable-pointer-cursor-for-buttons, pointer, null);

[XhmikosR]

Should we add cursor pointer?
Or a title or something. I feel like it's not clear one can interact with it.

@patrickhlauke: what do you think about this?

[MartijnCuppens]

Btw, I don't think we need to document what the color picker will look like, browsers might change this anytime.

I would add something about browser support (IE) though.

[XhmikosR]

I definitely agree with the above. See #29059 (comment)

[MartijnCuppens]

Ok, I've changed the copy to a notice about IE not supporting color inputs.