Users can create fields with reserved var bames

[fmontes]

Describe the bug User can add identifier and inode fields content types, with the variable name “identifier” or "inode". These variable names should be reserved.

To Reproduce Steps to reproduce the behavior:

1. Go to content types
2. Create or edit a content type
3. Add a text field with the name "identifier"
4. See that user can create it and the var name is "identifier"

Expected behavior Users should not be able to create those named fields.

Screenshots If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context Add any other context about the problem here.