Better base `ul` and `ol` styles

Turns out we were nixing the default browser spacing for bullets and
numbers, so this reverts that and instead just twerks the top/bottom
`margin`.
Remove explicit, unncessary `line-height` on `li` elements
Properly unstyle the `.list-unstyled` class by switching from `margin` to `padding`

Also add better comments
Better clearing on `.dl-horizontal` for empty `dd` elements

Instead of clearing the entire `.dl-horizontal`, we've moved the
clearfix mixin to the `dd` level to solve two problems in one: empty
`dd`s mid-list, and empty `dd` as the last child.

Addresses a few old issues, including #2824, #3819, #3821, #4062,
#6707, and #7180.

Sorry that took so long :D.