Enable Mattermost slash commands by default

Zeger-Jan van de Wegrequested to merge
zj-enable-slash-commands-mattermost into master

Slash commands are used for further integration with GitLab. This feature however, is disabled by default by Mattermost and also in gitlab-mattermost.

This commit will enable it by default, but only for new installs.

Fixes #1737 (closed)

Edited by GitLab Release Tools Bot

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