What version of Djblets are you using?
1.0.2
Which module(s) have the problem?
rbintegrations
What steps will reproduce the problem?

Create a Slack notification
Submit a new review request.

What is the expected output? What do you see instead?
Slack notification should be successful, but fails. If logging is turned on, a stackrace will appear in the logs.
What version of Python and Django?
Python 2.7, DJango 1.11.6
Please provide any additional information below.
Capturing network traffic using tcpdump revealed that the Content-Type header was being set incorrectly. The Content-Type header is set to application/x-www-form-urlencoded, but the body is json. Therefore, the request is refused by Slack (or Mattermost in my case).
The culprit is at line 189 of integration.py. urllib2 defaults to form data when posting. The fix is very simple. Change the line to read:
urlopen(Request(webhook_url, 'payload='+json.dumps(payload)))


Another option is to do this instead, as explained here:
json_data = json.dumps(payload)
urlopen(Request(webhook_url, json_data, {'Content-Type': 'application/json', 'Content-Length': len(data)}))