#2057 - Revise item name and slug validation - backslashes, refactor, error messages.

- disallowed backslashes in item validation.
- refactored the validation logic in the item model a bit.
- added no_backslash error messages in edit album/photo/movie forms.
- fixed error messages in add album forum (some missing, some text different from edit)
- added unit tests
- updated to v58 to correct any existing backslashes in item names