Slack wrong MR notification

ZD: https://gitlab.zendesk.com/agent/tickets/18317
GitLab: 8.6.1

Description

A Slack notification comes in after opening a MR about the MR being opened but it also comes in after a MR gets the minimum number of approvers and it uses the same message.

Next Steps

Correct message for MRs that get the minimum number of approvers

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/cc @brodock