Show attachment download progress, new stop button to cancel

Co-authored-by: Jamie Kyle <jamie@signal.org>

ts/test-both/processDataMessage_test.ts

@@ -100,9 +100,9 @@ describe('processDataMessage', () => {
     assert.deepStrictEqual(out.attachments, [
       {
         ...PROCESSED_ATTACHMENT,
-        chunkSize: 2,
         downloadPath: 'random-path',
         incrementalMac: 'AAAA',
+        chunkSize: 2,
       },
     ]);
   });