Return to share instance form after successful share.

Rather than a separate confirmation page after successfully sending
emails to share an instance, return to the share page in case you
want to share with another person.
Add a confirmation message to say that the invite has been sent.
Improve template for sharing an instance:

1) Call an instance of SayIt a SayIt
(suggestion from Martin)
2) Short description of what sharees can do.
3) Only a single invite is sent at a time (so make the text reflect this).
Send an email to an existing user on invite.

Also a bit of tidying for email templates, including getting the domain
correct in the links.
Some PEP8ification
More text on the accept invitation page.
Add a message to a newly invited user after setting their password.
Add link to share instance on home and add speech pages.

The templates in the sayit app don't have very many blocks at the moment,
which makes them rather hard to extend. The best I can see to do to for
adding this link is to put it at the top of the content block.

django-apptemplates allows us to extend the templates from speeches
without having to give them new names.