Change upload image button design

NOTE: This PR expects to merge `ajax-preview` PR first

Upload image link is previously shown by using a "button" typically, as a separate field in the submission form. However, the upload-image field is a substitute to add images in description.

This image button is changed to a text link written at the bottom of text editor as shown below

![upload-img-preview](https://cloud.githubusercontent.com/assets/932927/2944704/fdabad30-d9d7-11e3-9371-bf9679cf3f89.jpg)