Use Pajamas compliant clipboard button in Mattermost integration

What does this MR do and why?

Migrates buttons in app/views/shared/integrations/mattermost_slash_commands/_detailed_help.html.haml to the Pajamas compliant button.

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Relates to #416044 (closed)