The fact that we aren't dealing with the Django source tree anymore
allows us to drop several tearDown()/addCleanup() calls that were
concerned with removing apiece files/dirs/symlinks created by test
cases, as we are covered by the removal of the parent temporary tree
anyways.
Thanks Tim Graham for advice and review.
This allows makemessages/compilemessages tests in `test_extraction.py`
and `test_compilation.py` to actually run isolated from each other
(unaffected by stray FS objects left by cleanup actions failures, debug
sessions, etc.) and to take advantage of the parallel tests execution
feature like most of the Django test suite.
`test_percents.py` gets slightly refactored to not inherit from the new
machinery which sets up every test case to copy and run under a
temporary tree.
Refactored tests to use a sample project.
Updated extraction:
* Removed special handling of single percent signs.
* When extracting messages from template text, doubled all percent signs
so they are not interpreted by gettext as string format flags. All
strings extracted by gettext, if containing a percent sign, will now
be labeled "#, python-format".
Updated translation:
* Used "%%" for "%" in template text before calling gettext.
* Updated {% trans %} rendering to restore "%" from "%%".
