Fail unit submission if fuzzy flag is set

Unless users have administration privileges, now they're required to clear
the fuzzy flag.

This covers the backend side of evernote/pootle#157.

Conflicts:
	pootle/apps/pootle_store/forms.py
Require non-admin users to always clear the fuzzy flag

Non-admin users now have the fuzzy checkbox disabled by default. It'll be
shown if the initial state of the unit is fuzzy or some other action (MT
translation, auto-translation etc.) turned the unit into fuzzy state.

Whenever the fuzzy checkbox is displayed for non-admins, they are forced
to clear it, otherwise the submit button won't be enabled.

This covers the frontend side of evernote/pootle#157.
Django 1.5: fixed `UnitStateField` behavior

The interpretation of values changed in the templates and the field now is
getting values other than strings as well (`True`, `False` and `None`).

Conflicts:
	tests/forms/unit.py