[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)
(cherry picked from commit 33feed4723)
(cherry picked from commit 1ca21b95ff)

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