Custom fields, similar models missing export template support

[jeremystretch]

NetBox version

v3.0.9

Python version

3.8

Steps to Reproduce

1. Navigate to the list of custom fields
2. Click Export > Add export template
3. Attempt to select custom field as the content type

Expected Behavior

Custom field should be a valid option

Observed Behavior

Custom field (and others) are missing from the list. Looks like export template support was neglected when we moved the following models out of the admin UI in v3.0:

• Custom fields
• Custom links
• Export templates
• Tags
• Webhooks