Add flag to never delete the latest uploaded text
Difference detection in the project .lng files depends on comparing with the previous version. In translated languages, this version may get pushed out by translations, which means the strings of the next upload are considered to be new, even if they are in fact unchanged.
To prevent this from happening, texts should get a "precious" flag that ensures they are not deleted before the next upload.
Fix: Last uploaded change of each string is flagged as "last_upload" to ensure it never gets deleted, fixes #6314