Updates to conflict-finding code:
- made conflict-finding code in item model generate more user-friendly names (no longer random)
- renamed _randomize_name_or_slug_on_conflict to _change_name_or_slug_on_conflict since it's no longer random
- made conflict-finding code slightly more strict - foo.jpg and foo.png can no longer co-exist even if the slugs are different (a rare case anyway)
- removed conflict-finding code from item::move since it's duplicated in the item model