Adding Localization should migrate fields to defaultLocale instead of dropping them

[janus-reith]

Currrently, when adding localization, fields that now don't match the schema anymore will be dropped (Although there is a warning and it asks for confirmation).

I think it could highly increase DX to offer migrating the current values to the specifed default locale.
Not sure how complicated it is too hook into the current migration logic though.

[chaotic-justice]

need this too. currenlty using mongodb, after adding a locale i can fetch the original data by passing in locale: 'all'. switching to locale: 'zh' fails to default back to the original. the migrations doc is not clear on how to achieve this - migrating richtext data from locale-all to locale-others