Updating mattermost connector to use bot accounts

[btotharye]

Updated the mattermost connector to use bot accounts.

Proposed changes:

• Use version 2.2 of mattermostwrapper that allows for new token option
• Updated mattermost connector to use new token in the init call.
• Updated documentation to reflect new options for bot token.

Status (please check what you already did):

• added some tests for the functionality
• updated the documentation
• updated the changelog (please check changelog for instructions)
• reformat files using black (please check Readme for instructions)

[btotharye]

still have to add the changelog in, working on that

[btotharye]

@wochinge I can't remember who I was talking to about this, but this allows for using bot accounts now for mattermost which is a lot more secure so this all works and I just verified and updated the docs some.

Let me know if there are any questions, not sure who should review it.

[wochinge]

Thanks a lot, Brian!
@akelad Your squad or Prod / Loop?

[btotharye]

sorry didn't realize there were tests, I fixed those now as well.

[btotharye]

@wochinge I think there is an issue with the coveralls in the github actions since it got moved from Travis there is an error about the token. Just an FYI.

[akelad]

@wochinge you guys feel free to take that over