Allow grouping of messages based on email and name

[struan]

At the moment the /from/ page only uses email on to group messages from the same person on one page.

In some circumstances it may be useful to group based on name rather than email to avoid it being obvious that a set of messages were all sent by the same person.

We should provide an option to group by name and email for such circumstances. (We need both to stop people with the same name but different emails being grouped together)

This option should be off by default, and possibly not exposed in the admin interface as in most cases grouping by email is the correct thing to do.