Fix Mattermost alert webhook call

[Scotchester]

Mattermost seems to have gotten more particular about the Content-Type header of incoming webhooks in the upgrade from 5.18 to 5.22.

We were seeing errors of this kind:

{
  "level": "error",
  "ts": 1588614108.121396,
  "caller": "mlog/log.go:175",
  "msg": "Could not decode the multipart payload of incoming webhook.",
  "path": "/hooks/<hook_id>",
  "request_id": "wsb5g1xiqine7knmgj9yo9887w",
  "ip_addr": "<ip>",
  "user_id":"",
  "method": "POST",
  "err_where": "incomingWebhook",
  "http_code": 400,
  "err_details": "mime: no media type"
}

This change switches our alert webhook's requests.post call from using Requests' data param to using its json param, which sets the Content-Type to application/json, as Mattermost now requires.

See: https://requests.readthedocs.io/en/master/user/quickstart/#more-complicated-post-requests

Changes

• Switches Mattermost alert webhook requests.post call from data to json

Testing

In your cfgov-refresh virtualenv, open the Python console and walk through the following:

>>> import json
>>> import requests
# Be sure to replace the `GHE` in the string below.
>>> payload = {
...     'text': 'Manual MM webhook testing',
...     'username': 'alertbot',
...     'icon_url': 'https://GHE/raw/CFGOV/platform/master/alerts/scream.png'
... }

# Confirm existing error behavior:
# (Contact me for the full URL you should use.)
>>> resp = requests.post(
...     'https://mattermost/hooks/<hook_id>',
...     data=json.dumps(payload)
... )
>>> resp
<Response [400]>
>>> resp.content
b'{
    "id": "api.webhook.incoming.error",
    "message": "Could not decode the multipart payload of incoming webhook.",
    "detailed_error": "",
    "request_id": "b38mndqaz7y8zche3dtwjfj1zc",
    "status_code": 400
}'
# If you have MM system console access, you can also see an error in the logs there.

# Try the new behavior:
# (Contact me for the full URL you should use.)
>>> resp = requests.post(
...     'https://mattermost/hooks/<hook_id>',
...     json=payload
... )
>>> resp
<Response [200]>
# You will also see the successful post in the cf.gov Alerts channel.

Notes

• I think this may have the interesting side effect of actually using the scream emoji for the bot's avatar, instead of the stock webhook logo.

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows the standards laid out in the CFPB development guidelines
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Visually tested in supported browsers and devices (see checklist below 👇)
• Project documentation has been updated
• Reviewers requested with the Reviewers tool ➡️

[higs4281]

Go team 😱