Fix ItemPage editEntity, and test cases

Iba1fdce1cc199cda0b7f83febdf48ce6a11100e0 resulted in data being
normalised even when it was already normalized, making the data
unrecognisable to the Wikibase API.

The normalization routines have been moved from __normalizeLanguages
and __normalizeData to static methods with a single underscore prefix
so they can be easily accessed from unit tests.

Also fix docstrings to be pep257 compliant.

Bug: 69401
Change-Id: Ia2ff456c550670de924099c5993f109c676f9d39
Merge "Fix ItemPage editEntity, and test cases"