Reusable image uploading components, and use jQuery-File-Upload

This is a simpler and stronger version of pull request #407 (closed without merging).

* Create template upload_image.html for modal image upload panel
* Create reusable imageUploadPanel.js to control image uploading
* Create utility function save_uploaded_image()
* Raise modal panel using Bootstrap markup, not JavaScript
* Use jQuery-File-Upload for ajax uploading (it uses an iframe on IE9)
* Eliminate redirect after upload, and its error-handling mechanism