JSON keys are removed when pushing changes

[leonekmi]

Describe the bug
When you use placeholders in messages.json files from WebExtensions, they dissapear when you push changes.

To Reproduce
Steps to reproduce the behavior:

1. Create a project and a component based on WebExtension
2. Make some changes
3. Push these changes
4. placeholders keys are removed

Expected behavior
It should keep these keys to avoid compatibility issues with the extensions code

Screenshots
leonekmi/scrobbly@58ce628

Server configuration and status

weblate@hifumin:/srv/weblate$ ./manage.py list_versions
 * Weblate 3.3
 * Python 2.7.13
 * Django 1.11.17
 * Celery 4.2.1
 * celery-batches 0.2
 * six 1.10.0
 * social-auth-core 2.0.0
 * social-auth-app-django 3.1.0
 * django-appconf 1.0.1
 * translate-toolkit 2.3.1
 * Whoosh 2.7.0
 * defusedxml 0.4.1
 * Git 2.11.0
 * Pillow 4.0.0
 * python-dateutil 2.5.3
 * lxml 3.7.1
 * django-crispy-forms 1.6.1
 * django_compressor 2.2
 * djangorestframework 3.9.0
 * user-agents 1.1.0
 * jellyfish 0.6.1
 * pytz 2016.7
 * pyuca 1.2
 * python-bidi 0.4.0
 * PyYAML 3.12
 * tesserocr 2.4.0
 * Mercurial 4.0
 * Database backends: django.db.backends.mysql
 * Cache backends: default:RedisCache, avatar:FileBasedCache
 * Celery: redis://localhost:6379, redis://localhost:6379, regular
 * Platform: Linux 4.9.0-7-amd64 (x86_64)
weblate@hifumin:/srv/weblate$ ./manage.py check --deploy
System check identified some issues:

WARNINGS:
?: (security.W004) You have not set a value for the SECURE_HSTS_SECONDS setting. If your entire site is served only over SSL, you may want to consider setting a value and enabling HTTP Strict Transport Security. Be sure to read the documentation first; enabling HSTS carelessly can cause serious, irreversible problems.

System check identified 1 issue (0 silenced).

[nijel]

Bug in the translate-toolkit, proposed fix is here: translate/translate#3844

[nijel]

Thank you for your report, the issue you have reported has just been fixed.

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• In case you see similar problem, please open separate issue.
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