Add "accepted: invalid" or "accepted: incomplete" milestones to moderation workflow

[miketaylr]

Now that #3291 is done, this should be less messy to implement.

The idea is described in option 3 here:

add some common milestones like "accepted: invalid" or "accepted: incomplete". Once you change the milestone, we have the app close the issue, and put in some template comment on the public issue, and close that issue.

I think those 2 should be sufficient. The implementation would be to add some logic to process_issue_action to check the milestone and then probably add 1 or 2 methods to WebHookIssue to update the public issue (the rest should already be possible):

1. add some boilerplate message about being invalid or incomplete
2. change the milestone to invalid or incomplete
3. close the public issue
4. close the private issue

Somewhere around here:

webcompat.com/webcompat/webhooks/helpers.py

Lines 228 to 240 in 9038743

	elif (issue.action == 'milestoned' and scope == 'private' and
	issue.milestoned_with == 'accepted'):
	# private issue have been moderated and we will make it public
	try:
	issue.moderate_private_issue()
	except HTTPError as e:
	msg_log('private:moving to public failed', issue.number)
	return oops()
	else:
	# we didn't get exceptions, so it's safe to close it
	issue.close_private_issue()
	return make_response('Moderated issue accepted', 200)
	elif (scope == 'private' and issue.state == 'closed' and

[miketaylr]

Probably a good opportunity to move

webcompat.com/webcompat/webhooks/helpers.py

Line 281 in 9038743

def prepare_rejected_issue():

into WebHookIssue, as a staticmethod.