Follow-up to #2225681: Migrate D6 i18n blocks translated strings
Problem/Motivation
A migration for custom block (D6 boxes) translated strings is needed. The translations for the title and body of custom blocks is in the i18n strings module.
Proposed resolution
Write a custom source plugin. Note the translation for the title and body of custom blocks are on different rows in the locales_target source. The source plugin needs to keep track of what item is being translated on a row.
And, of course, tests.