- Resolves https://codeberg.org/forgejo/forgejo/issues/580
- Return a `upload_field` to any release API response, which points to
the API URL for uploading new assets.
- Adds unit test.
- Adds integration testing to verify URL is returned correctly and that
upload endpoint actually works
(cherry picked from commit 074413a2dc)
(cherry picked from commit 33feed4723)
