Unify definition panel styles

[domenic]

Currently, we are using the Bikeshed-generated .dfn-panel, which is styled via inline styles, on most specs. We are then using the Wattsi-generated .dfnPanel, which is styled via standard.css, on HTML.

We should instead be using a uniform class name, with styles in standard.css (and no inline styles). Probably the best path is to:

• Modify standard.css to style .dfn-panel instead of .dfnPanel. (Do not merge until the next step is complete... or maybe better, have a period where both are present.)
• Change HTML/Wattsi to use .dfn-panel.
• Revive https://github.com/tabatkins/bikeshed/pull/1287/files so that it removes the inline styling for .dfn-panel, instead letting standard.css take care of it.

[annevk]

There's a corresponding issue in that definition panels also come with script (script-dfn-panel), but we could tackle that separately probably.