- new vars for .lead size and weight (previously had none)
- new var for default margin-bottom on h1-h6 elements (previously had none)
- rearranged vars to combine two typography sections into one in variables.scss
- reassigned border-width var to hr-border-width
- made hr-border more specific as hr-border-color

- 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

- Fixes #13267 somewhat.
- Instead of a single, block-level class, let's use a combination of existing elements and classes.
- For block-level help text, use p.text-muted.
- For inline-level help text, use span.text-muted or small.text-muted.