Generate thumbnails when sending m.image and m.video messages. (#555)

* Send a thumbnail when sending a m.image

* Use the 'thumbnail_file' when displaying encrypted images

* Whitespace

* Generate thumbnails for m.video

* Fix docstring, remove unused vars, use const

* Don't change the upload promise behaviour

* Polyfill for Canvas.toBlob to support older browsers

* Lowercase for integer types in jsdoc

package.json

@@ -42,6 +42,7 @@
   },
   "dependencies": {
     "babel-runtime": "^6.11.6",
+    "blueimp-canvas-to-blob": "^3.5.0",
     "browser-encrypt-attachment": "^0.1.0",
     "browser-request": "^0.3.3",
     "classnames": "^2.1.2",