Fix save of duplicated default features in Designer

Steps to reproduce (10.10 + Web UI):
1. In Designer navigate to Tabs.
2. Copy the documentViewPage default feature.
3. Save. The progress bar spins forever and there is an error in the browser console:

Uncaught (in promise) TypeError: Cannot read property 'priority' of undefined

Designer seems to be trying to handle the copied feature as a default feature.

Reproducible for the following default features:

• Tabs (page)
• Drawer Items (browse)
• Drawer Items (page)
• Drawer Items (search)
• Drawer Menus (administration pages)
• Drawer Menus (pages)
• Buttons (results selection actions)
• Buttons (document actions)
• Buttons (results actions)
• Buttons (document create actions)
• Buttons (blob actions)
• Buttons (collection actions)