[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)

modules/structs/release.go

@@ -18,6 +18,7 @@ type Release struct {
 	HTMLURL      string `json:"html_url"`
 	TarURL       string `json:"tarball_url"`
 	ZipURL       string `json:"zipball_url"`
+	UploadURL    string `json:"upload_url"`
 	IsDraft      bool   `json:"draft"`
 	IsPrerelease bool   `json:"prerelease"`
 	// swagger:strfmt date-time