Wrong notification shown while creating merge request

[ayusht2810]

Description

When a user assigns a merge request to another user before creating the merge request, the assignee gets a notification of requested review instead of assigned merge request.

Steps to reproduce

1. Start creating a merge request on GtiLab.
2. Before creating a merge request, add an assignee to the merge request.
3. Create the merge request.
4. Check the notification received on the Mattermost.

Expected behaviour

Should show notification for assignment of the merge request

Actual behaviour

Notification received for the review request on the merge request

Additional context

This might be due to the fact that we are simply showing requested your review at line instead of checking whether the notification was for assignment of merge request or due to request review.

[ayusht2810]

Fixed by #471