Use the original `label` description list style

[nevans]

As a default for all description lists, the original "label" style is more readable.

This is slightly different from the original label dl though:

• slightly increase left margin for dd (to 1em)
• remove right margin on dd
• remove dt bottom margin and dd top margin, to reduce the gap between the term and its description to only the standard line-height gap

See the discussion at #1199. Because of that discussion, this PR gives both description lists the "label" style. A different PR (#1209) would also bring back distinct styling for the "note" list.

[nevans]

Here are some screenshots. Please note that I did add a second dt to both of the examples, because I think it's important to see how that displays too. I didn't add it to the examples, but I'm happy to add that to this PR or to make another PR for it.

v6.7.0:

current master:

This PR:

[nevans]

I'll make a separate PR to bring back a distinct "note" style, so that can be judged separately from this. 🙂 But I also want to tweak some of the styling issues that I dislike about the original, so I might not get to it until later today.