Rework the auto-login docs

I reworked the auto-login docs turning the instructions into lists of
steps and breaking up the text into an "enable" section and a "disable"
section. I think this is easier to read. It's certainly easier to
follow.

Further, I think this addresses the confusion around what happens if
you're not using "django_browserid.helpers.browserid_js" regarding
"browserid/autologin.js" since it explicitly addresses it in the
relevant areas as a discrete step.

Fixes #267 

r?
`browserid_js` gets displayed as the full import path here, but users don't use it that way. Giving it a short name and explicitly calling it a template helper would make this clearer, like 

```jinja
If you are not using the :py:func:`browserid_js template helper <django_browserid.helpers.browserid_js>`
```
(Assuming I got the syntax for that right)