Add using hash of photos to complicate procedure of parsing data.

Update {{photos}} table:
- add `hash` field.

Update <init_photos> migration:
- add `hash` field.

Update <Photo> model:
- updated common filename pattern: /static/photos/<block>/<id>/<size>_<hash>.<ext>.
- updated getFilename() and getWebPath() to use new pattern of filenames.
- implement beforeSave() to generate hash during saving new record.