Issues/364 more opengraph data over subdomains testcase

Note that this work is on top of https://github.com/mysociety/sayit/pull/400. It can probably be rebased to take that out if necessary.

This adds appropriate OpenGraph metadata to the following pages for an instance:
1) The instance home page
2) Speech detail pages
3) Speaker detail pages
4) Section detail pages

In the future it would be nice to have the section pages change their opengraph data when someone follows a link to a fragment to have details for the speech in the fragment.

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For the image here, and also in `speaker_detail.html` and `speech_detail.html`, I think it could be made more generic by using `request.scheme` instead of `http` and `request.get_host` instead of `request.META.HTTP_HOST` - just a bit of futureproofing :)