Add source_url support in preface.

If a <link/> is present in a preface/coverPage, assume that it is a
source URL and store it accordingly. Do not use this in the test for an
existing section, as it is quite possible it will change over time.
Strip tags from html in og_description for speech and section.

Fixes #416.
Fix display of section divider.

Tweak CSS so that the divider after a subsection on a section page
doesn't overlap the timeline, to match the divider between speeches.
Merge branch 'issues/354-source-url'
Merge branch 'issues/416-no-html-in-og-description'
Merge branch 'issues/428-tweak-section-divider'