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 "%%".
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