Build full set of updates and push them to backend just once

Currently translated at 70.9% (554 of 781)

Using class attribute seems more readable than self.get_file_format().

Currently translated at 70.7% (552 of 781)

As response handling has changed in Django 1.5, current approach no
longer works. In future we should use StreamingHttpResponse class, but
as this is available only since Django 1.5, it should be done once we're
ready to drop 1.4 support.

Since Django 1.5 CommandError is raised further instead of changing it
to SystemExist as was the case before.

Currently translated at 70.4% (550 of 781)

Using assertContains fails on Django 1.5 as it tries to convert response
into unicode string, what obviously fails with PNG data. Now PNG
responses can be checked with assertPNG, which tries to load the image
using Cairo.

Currently translated at 93.8% (15 of 16)

Currently translated at 70.0% (547 of 781)

Currently translated at 20.9% (163 of 781)

Currently translated at 100.0% (781 of 781)

Currently translated at 99.9% (780 of 781)

Currently translated at 100.0% (781 of 781)