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.