Added a Discord Integration option.

This allows Review Board to have an option of notifying Discord channels on

any Review Request changes. This commit also includes a refactoring of the

notify() function in SlackIntegration.

Added unit tests in discord/tests.py and they pass in RB3 virtual 

environment. RB4 has two exceptions (please see below). 
Tested it manually in reviewboard 3.0.x and 4.0.x.
Issues that still need to be addressed:   


Add assets (e.g. logos, trophies, etc.) at this location: 

https://static.reviewboard.org/integration-assets/discord


It has the same problem as the Slack and Mattermost Integration: 

two of the unit tests are complaining about this image here in RB4 -

/var/folders/f1/r8mpbfkd321804b47_tgytgw0000gn/T/rb-tests-EQOxMU/

static/rb/images/logo.png