Importing strings into AMOS makes it hard/impossible to edit them later

As discovered by nadavkav, this is a regression of recent AMOS storage migration.

Steps to reproduce

1. Let's have a component like qtype_geogebra with 2.7+ strings
2. The users imports translations and selects latest version 4.0+ during the import
3. Translations are backported so there is: 2.7+ original, 4.0+ translation and 2.7+ backported translation.

At this moment, AMOS does not allow to edit this 2.7+ translation easily, if at all.