Removing `Draft` status should send a notification to participants

Problem to Solve

When marking a merge request as ready (i.e. removing the DRAFT status) a notification is not triggered for the merge request. This means other users and services don't receive any kind of notification that the merge request is ready.

Proposal

Toggling of the Draft state to ready should trigger the appropriate notifications:

1. Email notification based on existing notification options:
   1. Watch: trigger a notification
   2. Participant: trigger a notification to participants of the MR

Original Description

## Summary When dealing with WIP in the title removing this doesn't send the normal notifications via email and other providers setup (E.g. HipChat) so no one knows you unmarked something as WIP.

Steps to reproduce

Make a Merge Request with an alternative person and mark it as WIP. Once you have made the MR and commented (These all send notifications as expected) unmark this as WIP. This adds it to the Merge Request log under the text: "Unmarked this merge request as a Work In Progress" - No notifications are sent.

Expected behavior

I would expect it to behave the same as making the initial merge request or commenting on the merge request and send notifications via email and other services setup.

Possible fixes

Add notifications in.