Allow external resources to be replaced

[fbacall]

Summary of changes

• Adds a setter similar to external_resources_attributes= except it removes any redundant external resources.
• When harvesting Bioschemas mentions, they use the above setter.

Motivation and context

Fixes #1029

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree
  to license it to the TeSS codebase under the
  BSD license.