Ref T12124. After D18134 we accept either "25" or "low" via HTTP parameters and when the field renders as a control, but if the form has a default value for the field but locks or hides it we don't actually run through that logic.

Canonicalize both when rendering the control and when using a raw saved default value.

• Created a form with "Priority: Low".
• Hid the "Priority" field.
• Before patch: Tried to create a task, was rebuffed with a (now verbose and helpful, after D18135) error.
• Applied patch: things worked.