Wrap report image file field in <div>

See crowdcover/crowdcover#360

The wrapping `<div>` is necessary to move the preview image outside of
the label for the file upload field.
Fix indentation of `.form-image` <label>
Use blank.gif as the default preview image
Show large version in overlay when clicking image preview

Closes crowdcover/crowdcover#360

When an image is displayed in the `img.report-image-preview` and the
`.report-image-preview` is clicked show the large version of
`.report-image-preview` in a modal overlay. This gives the user a better
idea of what the image will look like in the report.
Merge pull request #8 from bryanjswift/issue-360-modal-preview

Show image in modal overlay on preview image click