Filter only on enabled() where relevant

Fixes #3270 and should close off #3248 as well. Thanks, @julen!
Store: Use a fields whitelist instead of blacklist for UnitForm

Also see evernote/pootle@0d730852d9ec8e650779f9a46dfa06860e986c21
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()`.
Django 1.6: be explicit about the fields included in ModelForms
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 evernote/pootle#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.