Extract common fields from IssueType and EpicType into an Issuable concern

As mentioned in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10999#note_167226640, we should consider extracting any common fields from IssueType and EpicType into a concern, such as

    field :id, GraphQL::ID_TYPE, null: false
    field :iid, GraphQL::ID_TYPE, null: false
    field :title, GraphQL::STRING_TYPE, null: true
    field :description, GraphQL::STRING_TYPE, null: true