Both widgets need to use the same border width because of input groups.
Thus:
$btn-border-width   => $input-btn-border-width
$input-border-width => $input-btn-border-width

[skip sauce]

Refs #18253
[skip sauce]

- no real need for everything to be 100% shared
- padding looked and felt too large for inputs but not for buttons
- tying forms and buttons seems fine, but throwing in pagination feels wrong

Flagged upstream in bootstrap-sass by scss-lint

Fixes #11659, #12698, and #12349: Always apply position and z-index form controls in input groups for proper borders

Fix handling of btn-group placed inside input-group-btn (used for mixing dropdowns with normal buttons)

Fixes #11561: Add float: left; to .form-control within input groups to prevent IE9 from screwing up placeholder text and select menu arrows

Fixes #11351: Correct grid class reset on input groups by using attribute selector, not an old class from v3 betas

Instead of applying `.btn-` or `.input-` to individual elements in
button and input groups, we now have new sizing classes to keep your
markup a little leaner.

* Add `.input-group-sm` or `.input-group-lg` to your `.input-group` to
replicate `.input-sm` and `.input-lg`, respectively.
* Add `.btn-group-xs`, `.btn-group-sm`, or `.btn-group-lg` to your
`.btn-group` to replicate `.btn-xs`, `.btn-sm`, and `. btn-lg`,
respectively.

Both sets of new sizing classes simply use the existing individual
sizing classes as mixins, so customization of them is automatically
inherited when compiling.

In class names:
* large => lg
* small => sm
* mini => xs ("Extra small")

In screen size categories:
* Tiny => Extra small

move form layouts from input groups back to forms (messed this up in b281ad64)

align inline radio and checkbox groups with their label for horizontal forms, see http://stackoverflow.com/q/17990310/1596547 (Question 2)