models: Pull common email behavior into mixin

models: Pull common email behavior into mixin

A "Patch" and "Comment" share a common ancestor: emails. As a result
of this many fields are similar between the two. Rather than
duplicating code, pull the similar code out of the aforementioned
classes and into a Mixin.

This has the side effect of removing restrictions on Comment.content,
but this is probably OK: it's possible that someone could send an
empty mail in reply to a patch and we should show that.

Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Reviewed-by: Andy Doan <andy.doan@linaro.org>