Reorganize handlers and modules.

There should be not anything changed.
Add request handler to retrieve blob upload url.
Use 2 steps to upload event item.

The old method retrieve upload_url when page is loaded, and do real update
later. However, the generated upload_url may be out of date (invalid).
Now, just retrieve upload_url before ready to real upload.
Add field 'note' to coupon and preview type event.
Add macro for form <input> field.
Add macro for form <textarea> field.