Allow guests to set issue position on create

Update permission should not be required since we're not really updating
the sibling issues. Their relative position values may change if there
is not enough space and we need to shift some issues, but these numbers
are purely internal and does not change the position of the issue
relative to the existing issues.

app/services/issues/base_service.rb

@@ -74,9 +74,7 @@ module Issues
     def issuable_for_positioning(id, positioning_scope)
       return unless id
 
-      issue = positioning_scope.find(id)
-
-      issue if can?(current_user, :update_issue, issue)
+      positioning_scope.find(id)
     end
 
     def create_assignee_note(issue, old_assignees)

spec/services/issues/create_service_spec.rb

@@ -97,8 +97,6 @@ RSpec.describe Issues::CreateService do
         let(:issue_after) { create(:issue, project: project, relative_position: 50) }
 
         before do
-          project.add_reporter(user)
-
           opts.merge!(move_between_ids: [issue_before.id, issue_after.id])
         end