supporter: Reorganize form post-input notes.

This makes it easier to keep baseline alignment between labels and their
respective inputs. Declaring `margin-left: 51%;` for the post-input notes
is not exactly what we want, but it's much closer and less troublesome than
dealing with the vertical alignment of labels otherwise.

supporter: New form styling.

This makes better use of bigger screens, and degrades better to smaller
ones.

js: Render <video> inner HTML when no source is supported.

The HTML inside <video> is meant to be rendered by browsers that don't
support the tag at all. You have to respond to the JavaScript error event
to deal with browsers that support video, but no available source. See
<https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Using_HTML5_audio_and_video#Showing_fallback_content_when_no_source_could_be_decoded>;.

css: <dl>s clear each other.

This prevents <dl>s from stacking horizontally when their <dt>s are short.

blog: Headlines clear floats.

This avoids a situation where multiple summaries have floats that stack on
top of each other vertically, which end up getting far away from the actual
text.

In order to make this work, we had to change the styling of #mainContent on
blog pages (and others with a sidebar). Otherwise, headlines with
class="clear" would clear the sidebar as well.

contractpatch: Page layout adjustments.

Try to make sections clearer and avoid the "big monolith of text"
feeling.

css: Start a separate stylesheet for "big screen" styles.

I'm about to add more and I *suspect* this will be easier to maintain
in the long term, rather than duplicating the conditional many times.