Display icon for issues created by banned users on boards

Proposal

Frontend polish for #327355 (closed)

Similarly to how confidential issues are clearly indicated by the eye-slash icon on the issue list, issue board, and the issue itself, hidden issues should be indicated by the spam icon on the issue board.

Hidden issues are currently correctly indicated with the spam icon on the issue list, but not on the issue board.

Implementation plan

Issue board view

1. Add hidden field to GraphQL issue type:

    field :hidden, GraphQL::Types::Boolean, null: false, method: :hidden?,
          description: 'Indicates the issue is hidden because the author has been banned.'

2. Expose hidden attribute in app/assets/javascripts/boards/graphql/issue.fragment.graphql and ee/app/assets/javascripts/boards/graphql/issue.fragment.graphql
3. Add icon with tooltip to app/assets/javascripts/boards/components/board_card_inner.vue#L201
4. Update spec/frontend/boards/board_card_inner_spec.js