Django 1.6: removed `UnitStateField` field's validation step

It has been removed because in Django 1.6 now that's part of the
`BooleanField` field's `validate()` method, which is called in the field's
`clean()`.
Unit: forbid overwriting source texts from the web interface

Overwriting source texts in this context is undesired, which ended up in
mangled DB content because of #278.

The reasoning behind allowing to edit source texts can be explained with
the fact that terminology units have been sharing the editing/validating
form with regular units. It's not the case anymore — these are very
different contexts with different use-cases.

Fixes #277.