Fix the appearance of translatable pages diffs in TUX

1. Update the saving hook to save the correct revision ID:
the translation unit's ID instead of the whole page's ID.

2. Update the TranslationHelpers for the pre-TUX editor:
merge the function for translatable pages diff with
the regular page diff.

Done with help from Niklas.

Change-Id: Ie976c62a8d0f1b3217f7c9b29ac45c8bd89f9ccf
Merge "Fix the appearance of translatable pages diffs in TUX"