Include WIP status for MR webhooks in changes field

Problem to solve

Is not possible to know if the work_in_progress status changed using webhooks.

Further details

When changing the title of a MR to remove the WIP status, a webhook is sent (if configured) containing the following changes:

'changes': {
  'last_edited_at': {
    'previous': '2018-11-05 09:57:15 UTC',
    'current': '2018-11-05 10:19:44 UTC'
  },
  'title': {
    'previous': 'WIP:Update something something to git support for something',
    'current': 'Update something something to git support for something'
  },
  'updated_at': {
    'previous': '2018-11-05 10:19:36 UTC',
    'current': '2018-11-05 10:19:44 UTC'
  }
},

This information could be enough, but according to the work in progress docs, the WIP status can be also determined by the commits on the MR.

Proposal

Include work_in_progress changes in the changes section of the webhook.

What does success look like, and how can we measure that?

Ideally the information we would get in the webhook would be similar to:

'changes': {
  'last_edited_at': {
    'previous': '2018-11-05 09:57:15 UTC',
    'current': '2018-11-05 10:19:44 UTC'
  },
  'title': {
    'previous': 'WIP:Update something something to git support for something',
    'current': 'Update something something to git support for something'
  },
  'work_in_progress' {
    'previous': "True",
    'current': "False",
  },
  'updated_at': {
    'previous': '2018-11-05 10:19:36 UTC',
    'current': '2018-11-05 10:19:44 UTC'
  }
},

Links / references