Enhance Workflow of leaving editions

Summary

If appropriate, split into multiple issues

Tasks

• Refactor the leave confirmation into a popup rather than an entirely new page (similar to how deleting labs now is).
• The leave edition button should not be visible at all to people who are not allowed to leave the edition (utilise permissionservice for this)
• Similar to the #107 (closed) issue as there is an access denied error when returning "edition/{id}/leave" after the user had already left.