Collapse/summarize issuable system note assignee changes
When the assignee is changed in an issue or MR, the system notes can get out of control. We should summarize and/or collapse them as necessary to avoid situations like these: ![Screenshot_20190531-010327](/uploads/0a380ecee68f3223c3e59ceae079f3d3/Screenshot_20190531-010327.png)
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