[UI Framework] [K7] Update uiFramework build task and webpack config to support themed CSS files.

[cjcenizal]

@snide Here's an early birthday present for you! This adds a theme toggle to the UI Framework header.

[snide]

Awesome. This saves me a lot of time. Only thing I'm not sure of is whether there should be theme_dark/light variables when you already have a theme_dark.scss file. It's a little redundant. I think you could probably just get away with exposing the contents of that file in theme_dark.scss directly, which suddenly makes it a lot simpler to work with...

It's pretty minor and I'm not too concerned about it.

What theming obviously brings up is that there's no reason the docs themselves can't be written in the framework entirely at this point. Not the biggest issue for us, but I hate having to explain... "well, ignore this broken stuff, it's just coming from the documentation". Something to think about. Think it'd be a small half day project to convert.

[cjcenizal]

@snide I ended up having to create those variables files so that we could compile the documentation site CSS as a separate file from the UI Framework CSS. So the build task now generates four CSS files: doc site light, doc site dark, KUI light, and KUI dark.

I think building the doc site out of the UI Framework components is interesting and has some attractive benefits, but I'm also hesitant to do so at this stage because the way we're building the dark theme now will only work if we commit to completely overhauling Kibana to use the new components, and we're not quite there yet. So the dark theme system has a huge potential upside, but I don't think we're prepared to realize it yet -- but once we are, I think we should revisit!