#2035 - Centralize definitions of how thumb/resize filenames are generated.

- added graphics::convert_name_by_size() to convert an item's name/URL/path
for use as a thumb or resize image.
- revised item model to use new function.
- changed item::find_by_path to use new function instead of adhoc definitions.
It still requires making some assumptions, which are clearly documented.
For consistency (and without loss of efficiency), we now search for all
thumb/resize images without extensions.
- removed extra unit tests for the png file as it now follows the same case as jpg.