update "assign_editor" responder when assignee is not in team

[mpadge]

@xuanxu This should be a minor request. Our "assign_editor" responder is configured as per the 2nd example in docs:

...
teams:
editors: 1111111
...
responders:
assign_editor:
only: editors
...

buffy/config/settings-production.yml

Lines 106 to 109 in 93d01eb

	assign_editor:
	message: "The editor guide can be found at https://devguide.ropensci.org/editorguide.html#editorchecklist"
	only:
	- editors

How easy would it be to add an additional check whether assignee is already part of "editors" team, and issue an additional response if not? Ping @maelle