forms: Drag and drop uploads

This replaces the multi-file upload widget for a new-fangled drag and drop widget. The new widget supports a few super-powered features

  * Multi-file drag and drop
  * Multi-file select (for real this time)
  * Progress bar
  * Trash can for removal
  * Link for preview
  * Attachments can be disabled for the thread entry
  * Separate attachment fields can be added to any form
  * Attachment settings can be configured for each upload field
  * Attachments of a certain type can now be required

The widget is not connected to the Redactor editor, so that the editor can be disabled and the upload will still function properly.

### Outstanding
  * [ ] Internet Explorer 9 support (FileReader javascript class is required). More help [here](https://developer.mozilla.org/en-US/docs/Web/API/FileReader)
  * [x]  Integrate thumbnail and preview support for uploads
  * [x] Upload cancel button
  * [x] Fallback support for unsupported browsers (upload on submit)
  * [x] Remove outdated settings in Admin Panel
  * [x] Allow filtering of attachments by MIME type
  * [ ] Add nice file type icons

##### Upload Progress
![screen shot 2014-08-21 at 12 56 15 pm](https://cloud.githubusercontent.com/assets/672074/4000762/12a1e4f0-295d-11e4-918a-1e4ec7cbb4b8.png)

##### File list view with removal option
![screen shot 2014-08-21 at 12 56 41 pm](https://cloud.githubusercontent.com/assets/672074/4000768/1c6f1534-295d-11e4-9308-536564459788.png)

##### Thread entry settings
![screen shot 2014-08-21 at 1 05 55 pm](https://cloud.githubusercontent.com/assets/672074/4000865/035768a2-295e-11e4-8df9-a852855d1db1.png)

##### New file attachment field
![screen shot 2014-08-21 at 1 05 27 pm](https://cloud.githubusercontent.com/assets/672074/4000871/0db1146a-295e-11e4-944e-859254bfba4f.png)

##### Custom configuration for attachment field
![screen shot 2014-08-21 at 1 05 42 pm](https://cloud.githubusercontent.com/assets/672074/4000860/fce1ee70-295d-11e4-850a-dcfc3a02c6f6.png)