* Tweak the script to use `en-us` `.ftl` files as the source of truth.
* Update `ftl-tx` to a version that can handle referencing terms with arguments.
* Omit msg-ref-only strings from Transifex JSON
* Fix msg-ref-only strings not included in translated .ftl files
* Update ftl-tx. Simplify localize-ftl script.
* Tweak FTL -> JSON conversion to produce a single file
- Use locale directories for JSON files, since that's where the
Transifex client will interact with them
- Skip non-locale directories (e.g., don't create an .ftl file for
.DS_Store)
- Other minor simplification
