Sanitize HTML in some Story fields

Use the bleach package to sanitize HTML in a Story's ``title``,
``call_to_action`` and ``connected_story_prompt`` fields.

Don't escape HTML when displaying the connected story prompt.

Addresses #894
Try to fix html5lib dependency issues

bleach requires html5lib==0.9.5.  django-cms isn't picky about
the html5lib version.  Try putting bleach first in the requirements
file so a version of html5lib that works for both bleach and
django-cms is installed.

Addresses #894