Closes #12988: Introduce custom field choice sets (#13195)

* Initial work on custom field choice sets

* Rename choices to extra_choices (prep for #12194)

* Remove CustomField.choices

* Add & update tests

* Clean up table columns

* Add order_alphanetically boolean for choice sets

* Introduce ArrayColumn for choice lists

* Show dependent custom fields on choice set view

* Update custom fields documentation

* Introduce ArrayWidget for more convenient editing of choices

* Incorporate PR feedback

* Misc cleanup

docs/models/extras/customfield.md

@@ -79,9 +79,9 @@ Controls how and whether the custom field is displayed within the NetBox user in
 
 The default value to populate for the custom field when creating new objects (optional). This value must be expressed as JSON. If this is a choice or multi-choice field, this must be one of the available choices.
 
-### Choices
+### Choice Set
 
-For choice and multi-choice custom fields only. A comma-delimited list of the available choices.
+For selection and multi-select custom fields only, this is the [set of choices](./customfieldchoiceset.md) which are valid for the field.
 
 ### Cloneable
 

docs/models/extras/customfieldchoiceset.md

@@ -0,0 +1,17 @@
+# Custom Field Choice Sets
+
+Single- and multi-selection [custom fields documentation](../../customization/custom-fields.md) must define a set of valid choices from which the user may choose when defining the field value. These choices are defined as sets that may be reused among multiple custom fields.
+
+## Fields
+
+### Name
+
+The human-friendly name of the choice set.
+
+### Extra Choices
+
+The list of valid choices, entered as a comma-separated list.
+
+### Order Alphabetically
+
+If enabled, the choices list will be automatically ordered alphabetically. If disabled, choices will appear in the order in which they were defined.