adds "read only" flag to fields settings

[Marchino]

This pull request adds a read only flag so that admins can decide if a specific field_setting will be editable or not. It works with every kind of Field Type.

[a-barbieri]

Is there any migration that needs to be done in order to upgrade from prev versions?

[Marchino]

Yes! you need to add this migration to your application:

class AddReadOnlyToFieldSettings < ActiveRecord::Migration[5.1]
  def change
    add_column :binda_field_settings, :read_only, :boolean, default: false
  end
end