Rename redact_content option to include_content

The redact_content option never worked because it read the wrong config
section. The PR introducing it
(https://github.com/matrix-org/synapse/pull/2301) had feedback suggesting the
name be changed to not re-use the term 'redact' but this wasn't
incorporated.

This reanmes the option to give it a less confusing name, and also
means that people who've set the redact_content option won't suddenly
see a behaviour change when upgrading synapse, but instead can set
include_content if they want to.

This PR also updates the wording of the config comment to clarify
that this has no effect on event_id_only push.

Includes https://github.com/matrix-org/synapse/pull/2422

synapse/push/httppusher.py

@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
 # Copyright 2015, 2016 OpenMarket Ltd
+# Copyright 2017 New Vector Ltd
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -295,7 +296,7 @@ class HttpPusher(object):
         if event.type == 'm.room.member':
             d['notification']['membership'] = event.content['membership']
             d['notification']['user_is_target'] = event.state_key == self.user_id
-        if not self.hs.config.push_redact_content and 'content' in event:
+        if self.hs.config.push_include_content and 'content' in event:
             d['notification']['content'] = event.content
 
         # We no longer send aliases separately, instead, we send the human