[Multiple issue assignee] Fix a number of specs

app/models/concerns/issuable.rb

app/services/members/authorized_destroy_service.rb

app/services/system_note_service.rb

db/migrate/20170320171632_create_issue_assignees_table.rb

db/schema.rb

lib/github/import.rb

spec/features/dashboard/issuables_counter_spec.rb

spec/finders/issues_finder_spec.rb

spec/requests/api/v3/issues_spec.rb

spec/services/system_note_service_spec.rb

spec/services/users/destroy_service_spec.rb