Javascript string translation doesn't include settings.php overrides

Problem/Motivation

Setting string translations in settings.php via overriding "locale_custom_strings_en" (or whatever langcode is being used) does not work in Javascript via Drupal.t().

Steps to reproduce

Override a string by adding it to "locale_custom_strings_en" in settings.php.
Load the string in JS via Drupal.t().

Proposed resolution

Include string overrides via settings.php in the JS string translation system.

Remaining tasks

Fix the bug.
Add test coverage.

User interface changes

Drupal.t() usage will include strings overridden via settings.php.

Introduced terminology

TBD

API changes

TBD

Data model changes

TBD

Release notes snippet

TBD