Sync leave from room on Gitter to Matrix

Sync leave from room on Gitter to Matrix

Fix #2763 (closed)

Dev notes

$ set DEBUG=gitter:app:matrix-bridge:gitter-bridge&&npm start

gitter:app:matrix-bridge:gitter-bridge onDataChange {
  url: '/rooms/5fd024a437cfb32c719f7283/users',
  operation: 'remove',
  model: {
    id: ObjectID {
      _bsontype: 'ObjectID',
      id: <Buffer 5f 76 2f 95 98 6e 46 1e 66 30 59 d1>
    }
  },
  type: 'user'
}


gitter:app:matrix-bridge:gitter-bridge onDataChange {
  url: '/user/5f762f95986e461e663059d1/rooms',
  operation: 'remove',
  model: {
    id: ObjectID {
      _bsontype: 'ObjectID',
      id: <Buffer 5f d0 24 a4 37 cf b3 2c 71 9f 72 83>
    }
  },
  type: 'room'
}

Testing strategy

When the user is in the room:

1. Join a public room on Gitter
2. Send a message from Gitter
3. Observe the room on your favorite Matrix client
4. Leave the room from Gitter
5. Notice a leave event is sent to the Matrix room as well

When the user isn't in the room:

1. Join a public room on Gitter
2. Observe the room on your favorite Matrix client
3. Leave the room from Gitter
4. Notice nothing happens because the user wasn't in the room