Changed the way makemessages invokes xgettext from one call per
translatable file to one call per locale directory (using --files-from).
This allows to avoid https://savannah.gnu.org/bugs/index.php?35027 and,
as a positive side effect, speeds up localization build.
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 "%%".
Add the trimmed option to the blocktrans tag to trim any newlines and
whitespace from its content.
This allows the developer to indent the blocktrans tag without adding
new lines and whitespace to the msgid in the PO file.
Thanks to mpessas for the initial patch and Dmitri Fedortchenko for the
report.
