inbound_email: add support for signed email addresses

These utilities allow us to generate addresses of the form
`<localpart>+<integer>.<signature>@<domain>`, where the integer is the
primary key of a given object for which we want to track email
exchanges. The signature prevents the addresses from being forged, that
is, the addresses have to be explicitly generated and displayed by the
web app to be discovered.

The counterpart function retrieves all relevant email addresses from the
list of recipients of an email message, and validates the signatures to
recover the integer values that won't have been tampered with.

These new utilities are expected to be used in the views and signal
handlers pertaining to processing of inbound email messages.