Add Mattermost chat integration.

Review Request #9361 — Created Nov. 10, 2017 and submitted — Latest diff uploaded

Information

rbintegrations
master
5c9dd88...

Reviewers

This change integrates Mattermost with Review Board, so that when there
is activity on review requests, a message will be sent to the configured
Mattermost channel with the details, just like Slack.

All existing rbintegrations unit tests pass for SlackIntegration. Added
similar unit tests for MattermostIntegration, which also pass.

Manually tested navigating to the Integrations page on the UI. The new
form for Mattermost is displayed. Mattermost logo has been added, but
icon@2x needs to be created/uploaded still.