Prevent mass loss of translation metadata during sync

[mathjazz]

During sync, translation attributions, timestamps as well as unreviewed suggestions might get lost. That can happen in particular when strings are moved between files.

Since #3312 landed Pontoon should be able to gracefully handle resource migrations, meaning that translation authors, timestamps and suggestions are preserved if the localization file is moved to a different location or renamed.

In practical terms, this might be a dupe of #2014, but other solutions to the problem might be possible.