3099: Fix up loading spinners.

* Work around Rails asset pipeline bug by avoiding "-" in image filenames.
* Always use asset helpers: src="/assets/foo.gif" only works in dev mode.
* Center loading spinners.
* Use a "spinner" class to identify spinners in "remove spinner" code.
3099: Use appropriate tr>td markup if infinite scroll container is a table.
Remove unused class.
Merge branch '3099-spinner-assets' closes #3099