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]
- 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
