This was inspired by the discussion in Improve toggle button design and structure #28, and having a bit of
spare time and wanting to try something. This gets the "admonition"
part of those design ideas.

In the javascript setup, also configure the whole admonition title
element to be clickable. This happens only in the case where it is
a direct descendent of the admonition box and there is only one
admonition title descendent.

This is not designed to be perfect, but is designed to be mostly
safe with most themes. It still could cause problems, but it has
been tested with alabaster, sphinx_book_theme, and sphinx_rtd_theme.

I don't claim to know javascript or CSS, or have deep knowledge of
sphinx-togglebutton. But I do know enough and could read and web
search enough to see how to add the onclick watcher + data to
identify the right thing to toggle.

Suggested reviews:

• Someone that knows sphinx-togglebutton should see if this makes
  sense.
• Someone that knows javascript should see if this makes sense
• It would be prudent for someone else to test (I've tested the
  three themes above and firefox/chrome)
• But if no one can test the above, it seems fairly safe to me and
  could be released and fixed later.