FIX: 🐛 upload on IE9 wasn't working :'(

- FIX: make sure we set a default name to a pasted image only on Chrome (the only browser that supports it)
- FIX: use ".json" extension to uploads endpoints since IE9 doesn't pass the correct header
- FIX: pass the CSRF token in a query parameter since IE9 doesn't pass it in the headers
- FIX: display error messages comming from the server when there is one over the default error message
- FIX: HACK around IE9 security issue when clicking a file input via JavaScript (use a label and set `visibility:hidden` on the input)
- FIX: hide the "cancel" upload on IE9 since it's not supported
- FIX: return "text/plain" content-type when uploading a file for IE9 in order to prevent it from displaying the save dialog
- FIX: check the maximum file size on the server 💥
- update jQuery File Upload Plugin to v. 5.42.2
- update JQuery IFram Transport Plugin to v. 1.8.5
- update jQuery UI Widget to v. 1.11.1

config/locales/server.en.yml

@@ -1828,9 +1828,9 @@ en:
     pasted_image_filename: "Pasted image"
     store_failure: "Failed to store upload #%{upload_id} for user #%{user_id}."
     attachments:
-      too_large: "Sorry, the file you are trying to upload is too big (maximum size is %{max_size_kb}%kb)."
+      too_large: "Sorry, the file you are trying to upload is too big (maximum size is %{max_size_kb}KB)."
     images:
-      too_large: "Sorry, the image you are trying to upload is too big (maximum size is %{max_size_kb}%kb), please resize it and try again."
+      too_large: "Sorry, the image you are trying to upload is too big (maximum size is %{max_size_kb}KB), please resize it and try again."
       fetch_failure: "Sorry, there has been an error while fetching the image."
       unknown_image_type: "Sorry, but the file you tried to upload doesn't appear to be an image."
       size_not_found: "Sorry, but we couldn't determine the size of the image. Maybe your image is corrupted?"