Support assignee_usernames in IssuableFinder

Add CE negated params to issue resolver

- Add all negated arguments supported by IssueFinder
- Add resolver specs

Add EE IssueResolver GQL filter arguments

- Adds missing filter EE arguments to resolver
- Adds negated EE filter arguments supported by issue finder

Add EE changelog entry

Refactor EE issue_resolver_spec. Create less DB records

Add FOSS changelog entry