title: django admin: how to disable edit and delete link for foreignkey
body: <p>In django admin site, if the model has a foreignkey, by default it will be a select input and there are three links(edit, add, delete) like below.
  How to disable these links <strong>only for foreignkey</strong>?
  If using has_delete_permission, they it cannot be deleted in its own change form either.</p>
  <p>Default:
  <a href="https://i.stack.imgur.com/zQeqD.png" rel="noreferrer"><img src="https://i.stack.imgur.com/zQeqD.png" alt="default" /></a></p>
  <p>Expected:
  <a href="https://i.stack.imgur.com/BSRqN.png" rel="noreferrer"><img src="https://i.stack.imgur.com/BSRqN.png" alt="after" /></a></p>