Fix manage handlers ignoring multipart form bodies

The /manage page posts with FormData, i.e. multipart/form-data, but the edit,
delete, tags, restore and empty-trash handlers parsed with r.ParseForm, which
only reads urlencoded bodies. So every field came back empty: delete returned
200 with deleted:0 and moved nothing, edits saved nothing — while curl -d
(urlencoded) worked, which is why tests and manual checks missed it.

Parse with ParseMultipartForm (tolerating ErrNotMultipart so urlencoded still
works), via a shared parseForm helper used by all manage handlers. handleUpload
already did this. Also stop the UI reporting deleted:0 / updated:0 as success,
and add a multipart regression test.

trash.go

@@ -135,7 +135,7 @@ func (s *Server) handleRestore(w http.ResponseWriter, r *http.Request) {
 	if s.rejectCrossSite(w, r) {
 		return
 	}
-	if err := r.ParseForm(); err != nil {
+	if err := parseForm(r); err != nil {
 		http.Error(w, "bad request", http.StatusBadRequest)
 		return
 	}
@@ -175,7 +175,7 @@ func (s *Server) handleEmptyTrash(w http.ResponseWriter, r *http.Request) {
 	if s.rejectCrossSite(w, r) {
 		return
 	}
-	if err := r.ParseForm(); err != nil {
+	if err := parseForm(r); err != nil {
 		http.Error(w, "bad request", http.StatusBadRequest)
 		return
 	}