Add option to disable creation of teams

Description

Per https://gitlab.com/gitlab-org/gitlab-mattermost/issues/7 add an option in configuration file to disable creation of new teams. Eventually this setting should move into the database and then the Admin Console

QA Test Steps

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Mana

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Assignee

Corey Hulen

QA Assignee

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Reporter

Ian Tien

Epic Link

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Fix versions

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Mattermost Team

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Sprint

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GitHub Issue

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Components

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Configure