Fixes #15136.
Closes #15345.
Closes #15314.
Refs #14724, #14927.

[Fix issue #15719] Add a variable for .form-group's margin-bottom

fixes #15497: increase decimals on line-height-large from 1.33 to 1.3333333 to fix button sizing in Win 8.1 Chrome

border-radius CSS property doesn't affect <select>s in some browsers.
Hence, @input-border-radius doesn't affect <select class="form-control"> in some browsers.
Closes #15340.

[skip sauce]
[skip validator]

Added @gray-base variable to make @gray-{shade} reference, also allows @gray-base to be modified with less#modifyVars and the vars to cascade

In Sass, variable must be declared before it is used.
This also removes Miscellaneous section, because horizontal line color
and form / list paddings are Typography.

`@gray-light` darkens from #999 to #777, which puts `.text-muted` at the
threshold for the 4.5:1 WCAG minimum contrast[1]. #777:#fff is 4.48:1.

The “Example” headers in docs become #959595, which is contrast ratio
3:1, the minimum for larger text. Since the headers are less important
than the surrounding text, 3:1 is fine and an improvement on the
previous #bbb:#fff (1.92:1).

Fixes issue #13847.

[1] http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html

By definition, XS screens have no minimum size, only a maximum size.
(Symmetrically, LG screens have no maximum size, hence why there's no @screen-lg-max.)

change `@zindex-popover` and `@zindex-tooltip` values to be greater than
those set for `@zindex-modal` allowing tooltips and popovers to be
displayed in front of modals (and all other content).

* Removes default max-height from .navbar-collapse (so not every navbar
will get a max-height collapse section)
* Scopes regular 340px max-height to fixed top and bottom navbars only
(those are the only ones that really need a max-height to enable
scrolling)
* Adds a landscape media query for phones to cap the max-height at
200px for fixed navbars"