Refactor new tmx downloads view:
- consume a bit less memory in general (ex: 27MB instead of 29MB when selecting all repos for French, 14.5MB instead of 22MB when selecting only mozilla-central for Persian)
- don't put empty strings in tmx output
- use tabs instead of spaces in generated tmx (saves up to 2MB memory in file size and 0.5MB in memory consumption)
Merge pull request #289 from pascalchevrel/nobug-refactor-tmx-download-model-for-memory-savings

Refactor new tmx downloads view: