Fix EZP-23624: Unable to change base translation when editing content

JIRA: https://jira.ez.no/browse/EZP-23624

When editing content, it should be possible to base the current draft on an existing translation 
(/content/edit/object/version/EditLanguage/fromLanguage )

However, it seems the generated POST action was being handled too late in the process, and a number of checks are done before.
This handles the `FromLanguage` action, performing the redirect much sooner in the process.

Tests: Manual

See also: https://github.com/ezsystems/ezpublish-legacy/blob/master/kernel/content/edit.php#L562

Thanks, updated.