[Custom fields] Allow boolean values that only support one selection

### Description

There are several instances when you might want a boolean, single-item-selected value in custom fields (such as `Featured`). This granularity would help in development b/c you know that only one element will have that attribute.

### Steps to reproduce

n/a

### Environment

emdash: 0.31.0

### Logs / error output

```shell

```