Remove unnecessary dotted paths in email docs

[Mike Edmunds]

In docs/topics/email.txt, there are several references like this:

... :class:`~django.core.mail.EmailMessage` ...

But throughout that file, django.core.mail is the default module (due to the module:: django.core.mail directive at the top). This has exactly the same meaning, and is shorter (which would improve wrapping):

... :class:`EmailMessage` ...

The full dotted path is only necessary when you specifically want it to appear in the generated docs. And starting a reference with ~ character means "omit the full dotted path—just show the last component."

(Also applies to :func: and :meth: and any other inline Sphinx roles in email.txt. We could probably just remove~django.core.mail.—with the leading ~—throughout that file.)