v4: Forms cleanup

This branch aims to overhaul a decent amount of forms styles and documentation with the hopes of getting it near final. It's still WIP and has a few more todos (noted below).

Quick screenshot

Changes

• #17498 (closed), #18130 (closed): Removes horizontal padding on .form-control-label so it avoids overriding grid-based form layouts and renders correctly in stacked forms.
• #18515 (closed): Add .form-control-{sm/lg} classes to match small and large input classes, namely for vertical alignment of label and input in horizontal form groups.
• #18702 (closed): Updates form validation docs to include basic examples for situations in which you'd want to use each state.
• #18703 (closed): Drops .text-help for .form-control-feedback, designed specifically to only display temporary form validation feedback text in addition to normal help text.
• #18704: Updates form validation docs to include examples of validation feedback text alongside always-present form field help text.

Todos

• #17248 (closed), #17266, #19277: Fieldsets vs divs in form groups
• #18390 (closed): Consider renaming .radio and .checkbox
• #18573 (closed): Consider renaming .form-control-label for it's more contextual use cases (e.g., only horizontal/grid forms and not necessary with radios/checkboxes as shown in #18389).
• #19004 (closed): Color contrast and color blindness on validation colors Punting on this for another time
• #19139: Validation states on custom forms (needs select menus and file inputs)—maybe cherry-pick that PR into this one?
• #19771 (closed): Form control sizing not applying to <select>s.