"Add or Remove..." behaviour for PM creators

[JagWaugh (Andrew Waugh)]

Could be this again (or still):

Add/Remove user on private discussion not available when creator

This will be fixed by: https://github.com/discourse/discourse/commit/e76b598e0f63bc3a1c398a9293aba46d59ee92b1 Thx for detailed report, and sorry for the trouble.