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/sampleproject/update_catalogs.py

@@ -0,0 +1,60 @@
+#!/usr/bin/env python
+
+"""
+Helper script to update sampleproject's translation catalogs.
+
+When a bug has been identified related to i18n, this helps capture the issue
+by using catalogs created from management commands.
+
+Example:
+
+The string "Two %% Three %%%" renders differently using trans and blocktrans.
+This issue is difficult to debug, it could be a problem with extraction,
+interpolation, or both.
+
+How this script helps:
+ * Add {% trans "Two %% Three %%%" %} and blocktrans equivalent to templates.
+ * Run this script.
+ * Test extraction - verify the new msgid in sampleproject's django.po.
+ * Add a translation to sampleproject's django.po.
+ * Run this script.
+ * Test interpolation - verify templatetag rendering, test each in a template
+   that is rendered using an activated language from sampleproject's locale.
+ * Tests should fail, issue captured.
+ * Fix issue.
+ * Run this script.
+ * Tests all pass.
+"""
+
+import os
+import re
+import sys
+
+proj_dir = os.path.dirname(os.path.abspath(__file__))
+sys.path.append(os.path.abspath(os.path.join(proj_dir, '..', '..', '..')))
+
+
+def update_translation_catalogs():
+    """Run makemessages and compilemessages in sampleproject."""
+    from django.core.management import call_command
+
+    prev_cwd = os.getcwd()
+
+    os.chdir(proj_dir)
+    call_command('makemessages')
+    call_command('compilemessages')
+
+    # keep the diff friendly - remove 'POT-Creation-Date'
+    pofile = os.path.join(proj_dir, 'locale', 'fr', 'LC_MESSAGES', 'django.po')
+
+    with open(pofile) as f:
+        content = f.read()
+    content = re.sub(r'^"POT-Creation-Date.+$\s', '', content, flags=re.MULTILINE)
+    with open(pofile, 'w') as f:
+        f.write(content)
+
+    os.chdir(prev_cwd)
+
+
+if __name__ == "__main__":
+    update_translation_catalogs()