Fixed #24257 -- Corrected i18n handling of percent signs.

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 "%%".

tests/i18n/commands/locale/fr/LC_MESSAGES/django.po

@@ -17,55 +17,5 @@ msgstr ""
 "Content-Transfer-Encoding: 8bit\n"
 "Plural-Forms: nplurals=2; plural=(n != 1)\n"
 
-#. Translators: Django template comment for translators
-#: templates/test.html:9
-#, python-format
-msgid "I think that 100%% is more that 50%% of anything."
-msgstr ""
-
-#: templates/test.html:10
-#, python-format
-msgid "I think that 100%% is more that 50%% of %(obj)s."
-msgstr ""
-
-#: templates/test.html:70
-#, python-format
-msgid "Literal with a percent symbol at the end %%"
-msgstr ""
-
-#: templates/test.html:71
-#, python-format
-msgid "Literal with a percent %% symbol in the middle"
-msgstr ""
-
-#: templates/test.html:72
-#, python-format
-msgid "Completed 50%% of all the tasks"
-msgstr ""
-
-#: templates/test.html:73
-#, python-format
-msgctxt "ctx0"
-msgid "Completed 99%% of all the tasks"
-msgstr ""
-
-#: templates/test.html:74
-#, python-format
-msgid "Shouldn't double escape this sequence: %% (two percent signs)"
-msgstr ""
-
-#: templates/test.html:75
-#, python-format
-msgctxt "ctx1"
-msgid "Shouldn't double escape this sequence %% either"
-msgstr ""
-
-#: templates/test.html:76
-#, python-format
-msgid "Looks like a str fmt spec %%s but shouldn't be interpreted as such"
-msgstr "Translation of the above string"
-
-#: templates/test.html:77
-#, python-format
-msgid "Looks like a str fmt spec %% o but shouldn't be interpreted as such"
-msgstr "Translation contains %% for the above string"
+msgid "year"
+msgstr "année"