Fix commit info for embedded argument widgets

When loading data via API, use the commit information bundled inside
each argument, rather than a `commit` root property.
Fix styles for embedded widgets
Add css namespace for argumenta styles
Use standard namespacing for widget styles
Fix header styles for embedded widgets
Use ribbon icon for tag counts

Technically, this is the bookmark glyphicon.  It's visually nicer than
the tag glyphicon, and Argumenta's tags are a bit unconventional
anyhow.
Use icon in tag headers
Refine tag count appearance
Refine tag icon colors
Fix proposition tag counts test
Change label on user widget to 'Arguments'
Make proposition dragging easier

This allows propositions to be dragged by a clicking anywhere on the
main panel, including text and whitespace.

Selecting text is now more difficult, but can be done by clicking
outside the proposition and dragging your selection inside.

If this is a significant issue, a "Copy text" action might be
worthwhile.