mod_push: Optionally include message sender/body

mod_push: Optionally include message sender/body

Add 'include_sender' and 'include_body' options.  If one or both of them
are set to 'true', a urn:xmpp:push:summary form with the enabled
field(s) is included in push notifications that are generated for
messages with a body.

The 'include_body' option can instead be set to a static text.  In this
case, the specified text will be included in place of the actual message
body.  This can be useful to signal the push service whether the
notification was triggered by a message with body (as opposed to other
types of traffic) without leaking actual message contents.