api/metadata: Fix issues detected with hypothesis

Running metadata tests with multiple hyothesis examples uncovered those
issues in api-1-raw-extrinsic-metadata-swhid Web API view:

  - ExtendedSWHID.from_string must be used to parse extended SWHID.

  - link-next URL for pagination was invalid.

  - next_page_token must be encoded before providing it to urlsafe_b64encode.