The translation of sections edited by VE are saved. They need to be restored against its source sections when translation is resumed.
[x] The basic algorithm is to match against the section identifiers.
 If source article changed heavily, we need to use a better algorithm to identify source sections even if it changed.
 And fallback to original source revision if source article changed beyond restoring a saved translation.
[x] Status message about fetching the saved translation should be shown to translator
 Status message about restoring the fetched translation should be shown to translator