Fix leaving of Mattermost channels

[roosterfish]

Overview

This MR fixes the ability to leave Mattermost channels unsing the !room leave command.

Changes

• Align the driver call for leave()
• Add a new @property field which yields the User ID of the Bot account
• Add the channel ID when logging the join event
• Remove a not required variable assignment

[Vaelor]

Thanks a lot @Roosterfish