Better og stuff for speech detail page.

* Move the title generation to the view so that we can easily use it for
both the og_title and the page_title.
* Change to using bleach rather than vanilla striptags.
OG stuff for a section.
Check for correct looking og_image urls.

Check that the speech and speaker detail views give sensible
looking og:image urls.

Currently these images are being created in an uploads directory
at the top level of the django-sayit checkout for every test run
(along with imager from speaker_tests.py) which is obviously less
than ideal.
Override MEDIA_ROOT as well as MEDIA_URL.

This has two desirable effects:

1) The files created by the tests are in a temp directory which is
deleted at the end, so don't end up cluttering up the real MEDIA_ROOT
directory.

This could be done better with a context manager - probably in a class
decorator, but the temporary directory should be deleted unless things
go wrong.

2) FileSystemStorage keeps its own copy of MEDIA_URL, so
override_settings(MEDIA_URL=...) doesn't make all the tests pass all
the time - exactly which ones fail and when seems to vary depending on
the system.
Skip tests which need override_settings(MEDIA_ROOT) in Django 1.4.

override_settings(MEDIA_ROOT) isn't made to sort out FileSystemStorage
until Django 1.5: (https://github.com/django/django/commit/a24ffa52d029)
so let's skip tests that need that in 1.4.

In speaker_tests, instead of skipping the whole test, which does other
useful stuff, I've just put an if around an assertion.