Cleanup $input SCSS variables

Cleanup $input- SCSS variables from _variables.scss and replace usage with appropriate GitLab UI design token or variable.

$input-height: 32px;
$input-danger-bg: #f2dede;
$input-group-addon-bg: $gray-10;
$gl-field-focus-shadow: rgba(0, 0, 0, 0.075);
$gl-field-focus-shadow-error: rgba($red-500, 0.6);
$input-short-width: 200px;
$input-short-md-width: 280px;
$input-md-width: 240px;
$input-lg-width: 320px;

1. Remove static hex color values for suitable GitLab UI colors
2. Remove rgba() color functions
3. See if we can remove variables in general