Adds enablePublicSharedBoards to config.json

[mcspud]

This PR was approved, but the commonly used docker compose files where missed.

This PR adds the enablePublicSharedBoards flag into the config file for consistency; hopefully users like me aren't questioning their sanity when trying to stand this up and test it for the first time 😉

[mattermod]

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[mattermod]

This PR has been automatically labelled "stale" because it hasn't had recent activity. A core team member will check in on the status of the PR to help with questions. Thank you for your contribution!

[sbishel]

@mcspud - Thanks for the contribution

Please help complete the Mattermost contribution license agreement?

And we can get this merged.