R1:eeec726ded2b

Remove all current Authors / Reviewers from "CCWelcome" mail

Summary:
We'll incorrectly send CCWelcome mail to users who would be added as CCs but are blocked by the new "$dont_add" stuff, for
example when a revision is updated and the user has a Herald rule which triggers them getting CC'd. See D2057.

Potentially a better fix for this would be to have "addCCs" return a list of the CCs it actually added, rather than duplicating the
logic of removing CCs in two places. However, that's not trivial since…