[GITEA] Add Upload URL to release API

- 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)

models/repo/release.go

@@ -132,6 +132,11 @@ func (r *Release) HTMLURL() string {
 	return r.Repo.HTMLURL() + "/releases/tag/" + util.PathEscapeSegments(r.TagName)
 }
 
+// APIUploadURL the api url to upload assets to a release. release must have attributes loaded
+func (r *Release) APIUploadURL() string {
+	return r.APIURL() + "/assets"
+}
+
 // Link the relative url for a release on the web UI. release must have attributes loaded
 func (r *Release) Link() string {
 	return r.Repo.Link() + "/releases/tag/" + util.PathEscapeSegments(r.TagName)