Extract issuable assignee filters to separate class

- Issuable finder does not handle assignee_usernames, only assignee_username
- IssueResolver arguments checks mutually exclusive arguments assignee_username(s)
- IssueResolver handles plural version of assignee_username filter

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

This moves author filtering related code to a separate class

- **app/finders/issuable_finder.rb:**

Adds note about how IssuableFinder is virtual.

- **app/finders/issuable_finder/params.rb:**

Ignore `release_tag` when searching on a group.

- **changelogs/unreleased/
  hide-inoperable-group-search-releases-filter-backend.yml**

Changelog.

- **ee/spec/finders/issues_finder_spec.rb:**
- **spec/finders/issues_finder_spec.rb:**

Adds test for filtering by group and release.

- **spec/support/shared_contexts/finders/
  issues_finder_shared_contexts.rb:**

Adds a release to the test suite.

Allow for issue boards scoping to an iteration. For that we need to
store the iteration to which the board is scoped into iteration_id
on boards table. This change also allows issue filtering on CURRENT
iteration, that is being calculated based on Date.today

This moves the core logic of the merge request resolver and the issues
resolver into a finder (technically a composition of finders), so that
we can re-use it in other resolvers/mutations without needing to
instantiate an entire resolver and run the full resolver life-cycle each
time.

Speed up NOT Issue filters by avoiding subqueries as much as possible.
Since things were getting unwieldy I also refactored methods into an
IssuableFinder::Params class that clearly separates the finder methods
from param helper methods.

These classes clearly separate the finder methods from param helper
methods, making the IssuableFinder and IssuesFinder classes much
cleaner.