Add support for filtering by negated `current` iteration in GraphQL

Related to discussion #299504 (comment 494630709)

!48040 (merged) added support for filtering by current iteration, so user can list "issues in the current iteration", but it would be nice to be able to also negate this parameter so user can list "issues not in the current iteration".

UPDATE: All negated issue filters supported by the IssueFinder where added to GraphQL and also some non-negated that were missing in both project/group IssueResolver and Boards IsseResolver.