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'
Capitalise labels automatically taken from model.

By default, a model form field would capfirst() the verbose_name, so we
should do the same.
Harmonize capitalization in links.
Show header buttons on all speakers page if there are no speakers.

We still want people to be able to import speakers if there aren't
any already, in fact, we particularly want them to be able to!

The nicest way I can think to do this is to include the normal buttons
from the header. It seems a bit cluttered with the 'All Speakers' heading
though, so I've taken that out until there are some.
Move instance home buttons to sayit.

These buttons used to live in the template in sayit.mysociety.org,
for reasons no-one can remember.