Allow filtering of issues by confidentiality in GraphQL

Summary

It's possible to filter issues by various attributes; such as epicId, milestoneTitle and assigneeUsername (among others). In order to fully support issue lists in Vue we need to support filtering by confidentiality state in the GraphQL API.

It doesn't seem to make sense for the confidential attribute to have None/Any filtering or negated filtering as it is a boolean yes/no value, so we don't have to implement them.

Acceptance criteria

• User can filter by confidential attribute