Variables for media queries

[printercu]

This allows use media queries like this:

@media @sm-only {
}
@media @md-up {
}

It also adds screen for all current media queries. I can remove it, if you think it's not necessary.

[cvrebert]

It also adds screen for all current media queries.

Why?

[printercu]

I've found #13014 is for the same purpose. Seems like this implementation is almost the same, except it'll takes less space in sass.

[printercu]

Indeed i took the idea and some code from foundation, then adopted it for bootstrap :/. It also had screen rule.

[printercu]

Maybe it'll be better to remove this rule to be able to use this rules for other medias. And markup will remain responsive for all medias. I'll remove it right now.

[cvrebert]

Also, landscape & portrait aren't used anywhere.

[cvrebert]

I suspect that we'll end up preferring #13014, since it doesn't involve using raw string interpolation.

[printercu]

I updated all media queries in project. What do you think about @float-breakpoint-up, @float-breakpoint-blow? There is one point whre @xs-below can be used https://github.com/twbs/bootstrap/blob/master/less/navbar.less#L95 Should I add this value?

[printercu]

Sorry, missed. So should i close PR in favor of #13014?

[printercu]

I think this method is more readable and follows less surprise principle.

[cvrebert]

@mdo will decide between this PR and the other PR.

[cvrebert]

@screen-xs-min is absurd to begin with; don't bother trying to accommodate the one media query that involves it.

[mdo]

Punting on this one.