Prevent user adding new values in the enum (i.e. tag) fields

[demeringo]

Bug description

When editing a field that is expected to offer a single choice in a list (value picker), user can declare a entirely new value (that is not in the existing enum).

In this case, behavior is strange: It seems to remove the status header from the csv (?) and submit the new value (as expected).

To Reproduce

This this PR where I tried to add an arbitraty value for status directly via the Form.

#73

Expected behavior

Use this setting on the field widget configuration.

{
      "name": "orga",
      "subtype": "tag",
      "allowNew": true,
      "bgColor" : "#F0BE86"
    },

JSON OUTPUT

Additional context