[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)
(cherry picked from commit 874f07cec2)
(cherry picked from commit a78c538d8e)
(cherry picked from commit bef38ce382)
(cherry picked from commit c8eb345354)

models/repo/release.go

@@ -133,6 +133,11 @@ 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)