Fixes https://openhatch.org/bugs/issue938 - strange result from canceling an openid request

What I did: Updated the authopenid/signin.html template to reflect the account/login.html template so that django_authopenid displays the correct template when the user cancels the OpenID signup.

I considered several ways to do this:
1. Update the template referred to by the signin functions in django-authopenid/views.py so that the expected template is shown (this pull request)
2. Change the template used by the signin functions in django-authopenid/views.py to use 'account/login.html' (seemed a bit messy to be changing 3rd party files)
3. Copy the functions in django-authopenid for signin into mysite/account/views.py and tell them to use the 'account/login.html' template instead (seemed like too much redundancy)

I'm new to django and web dev in general, so there's probably another way to do this that I've missed. I think OH is an awesome idea though, and I really want to help out, so please suggest feedback and I will make the necessary changes :)