Use client-side polling (instead of server-side) polling when using the importer upload.
fixed the pluralization of translated strings
Merge pull request #1906 from menegon/master

fixed the pluralization of translated strings
Merge pull request #1899 from garnertb/uploader-clientside-polling

Use client-side polling (instead of server-side) when using the importer upload.
Initial celery integration.
Add notification support.
Add email support.
Add update layers.
Add a celery app.
Flake8 fixes.
Create thumbnails asynchronously.