Instead of the regular border-radius var, use the input ones to match sizing with their associated controls.

When using a <label> as an .input-group-addon, there will be a default margin-bottom of .5rem via Reboot.
This will lead to undesirable whitespace below the label and the <input> will become taller than needed. By removing the margin, it will play nice with <label> elements.

Closes #19012

1. Invert `:first-child` into `:not(:last-child)` and vice versa
2. Remove double border at the left of `.form-control`
3. Shifts negative margins of `.btn` and `.btn-group` to retain
rightmost border when using at the left of `.form-control`.

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.