#1946, 1947 - Make altered names/slugs more user-friendly, make conflict-finding code check filenames with no extensions
- Reduced from four places that alter names/slugs to two (one to populate empty slugs, one to handle conflicting names/slugs).
- For empty slugs, fill with generic human-readable name (e.g. "photo") instead of random value.
- For conflicting names/slugs, add suffix that's sequential (e.g. "-01"), only using random after 99 conflicts.
- Made conflict-finding code check filenames with no extensions.
- Renamed _randomize_name_or_slug_on_conflict to _check_and_fix_conflicts.
- Added unit tests.  Also cleaned up existing unit tests to reflect new logic and removed a redundant test.
- Added installer logic to correct existing items now considered in conflict.
- Revised gallery_task to look for duplicate names based on new criteria.