this is a resubmission of #5875, but now against branch 3.0.0-wip
does not exclude elements under a fieldset's legend
issue #6058 is a duplicate of #5605

* Deprecate .input-append and .input-prepend
* Use new, single base class for component, .input-group
* Deprecate .addon for .input-group-addon
* For compatibility with all buttons, require .input-group-btn to wrap buttons and button dropdowns
* Still need to reimplement with segmented button dropdowns, but that's dependent on a refactor of those first

remove .border-radius mixin and replace with standard border-radius property, since -webkit-* and -moz-* are only needed for older versions

add missing icon class definition for buttons:
'[class*=" icon-"]'

add new vars for common border-radius, upping border-radius of inputs to 4px; add border-radius changes to -large, -small, -mini for buttons and paginations

add new vars for common font-sizes and paddings with large, small, and mini components; resize pagination and buttons to better match each other

change up box-shadow mixin usage everywhere to use escaping again, or local var if necessary for further var evaluation

fixes #4445: .btn-link shouldn't have a pressed button-like state at all, so nuking everything on :active as well