records: display text-based resources in preview pane

[alisonrclarke]

The modal that displays images for each table could also display text-based resources such as scripts or text files. This is partially implemented in code_view_widget.html (shortly to be moved to a separate javascript file as part of the work for #138) but currently preview_location, which used to determine whether to include the link to the modal in render_associated_files, is only added to images in the python code.

We should also add syntax highlighting for code where possible.