Add support for customizing the evergreen URL slug

### Description

Currently, when the "Evergreen URL" is toggled on for a post, the date is replaced with a hard-coded `feature` string. We should add functionality that allows editors to customize the string that will replace the date in the URL. We could allow a text field and/or a term selector. We will need to verify that redirects work properly in the event that the string is changed on a post.

### Use Case

An editor should be able to customize the string that replaces the date in the evergreen URL.