Update Mattermost to 5.37.1 and add Focalboard plugin support

What does this MR do?

New features in Mattermost 5.37 include Emoji 13.0 support (up from Emoji 4.0), better skin tone support for emojis, a beta for Collapsed Reply Threads, performance improvements, and the addition of some previously Mattermost EE-only Incident Collaboration features to Mattermost TE. There's a lot this time. The full changelog is here as usual.

Also, this updates the Nginx template used by Mattermost to add support for Focalboard (see https://mattermost.atlassian.net/browse/MM-37058 and #6173 (comment 620512411) for more info). I also added an extra block that's supposedly required for websocket connections according to our documentation, but it's never been there before.

@amyblais @jasonblais @twk3

Checklist

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
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Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
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