Allow Customization of GitLab Issue creation

## Summary With the deprecation of the plugins, and the introduction of the new global integration, we've lost two really useful goodies on the issue creation form, namely the assignee, and the labels. Since this does not affect the old plugins, I've chosen to file it as a feature request (for the global integration) rather than a bug. ## Motivation This improvement would allow a sentry user to completely create an issue -in the cases where workflow doesn't demand more than an assignee and labels- straight from the sentry interface, without having to scoot over to each project's issue tracker in GitLab. ## Additional Context How it was with the deprecated plugins:  How it is with global integration: