Add kwargs to Message constructor in Mattermost connector

[davdr]

In #1116 the user_id arg was added to all events, and the event constructors were updated to use kwargs. However the Mattermost connector was missed and so the positional arguments are now broken.

This PR switches the Mattermost connector to use kwargs instead.

Fixes #1773

[codecov]

Codecov Report

Merging #1774 (00ecc34) into master (bd61d9a) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1774   +/-   ##
=======================================
  Coverage   99.27%   99.28%           
=======================================
  Files          78       78           
  Lines        4583     4584    +1     
=======================================
+ Hits         4550     4551    +1     
  Misses         33       33           

Impacted Files	Coverage Δ
opsdroid/connector/mattermost/__init__.py	100.00% <ø> (ø)
opsdroid/connector/slack/connector.py	100.00% <0.00%> (ø)

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[FabioRosado]

Hello @davdr apologies for the delay in reviewing this PR. This looks good, but it seems there are some linting issues. Are you able to fix them with black?

[davdr]

Thanks @FabioRosado -- should be okay now.

[FabioRosado]

Thank you for fixing the lint issues, the text failure seems a bit weird but isn't related to this PR so I'm accepting it and merging it.

Also thank you for creating the issue and fixing it 😄 👍