i18n: Rename email subject files so i18n detects them.

Apparently, when we renamed these files to no longer have a .txt
extension, we accidentally removed them from the set of strings for
translation, because `manage.py makemessages` by default only
processes .txt and .html files under the templates/ directory.

Fix this by adding a .txt extension.

docs/subsystems/email.md

@@ -12,7 +12,7 @@ On to the documentation. Zulip's email system is fairly straightforward,
 with only a few things you need to know to get started.
 
 * All email templates are in `templates/zerver/emails/`. Each email has three
-  template files: `<template_prefix>.subject`, `<template_prefix>.txt`, and
+  template files: `<template_prefix>.subject.txt`, `<template_prefix>.txt`, and
   `<template_prefix>.source.html`. Email templates, along with all other templates
   in the `templates/` directory, are Jinja2 templates.
 * Most of the CSS and HTML layout for emails is in `email_base.html`. Note

zerver/lib/send_email.py

@@ -57,7 +57,7 @@ def build_email(template_prefix: str, to_user_ids: Optional[List[int]]=None,
     })
 
     def render_templates() -> Tuple[str, str, str]:
-        subject = loader.render_to_string(template_prefix + '.subject',
+        subject = loader.render_to_string(template_prefix + '.subject.txt',
                                           context=context,
                                           using='Jinja2_plaintext').strip().replace('\n', '')
         message = loader.render_to_string(template_prefix + '.txt',

zerver/tests/test_templates.py

@@ -83,18 +83,18 @@ class TemplateTestCase(ZulipTestCase):
             'zerver/delete_message.html',
         ]
         unusual = [
-            'zerver/emails/confirm_new_email.subject',
+            'zerver/emails/confirm_new_email.subject.txt',
             'zerver/emails/compiled/confirm_new_email.html',
             'zerver/emails/confirm_new_email.txt',
-            'zerver/emails/notify_change_in_email.subject',
+            'zerver/emails/notify_change_in_email.subject.txt',
             'zerver/emails/compiled/notify_change_in_email.html',
-            'zerver/emails/digest.subject',
+            'zerver/emails/digest.subject.txt',
             'zerver/emails/digest.html',
             'zerver/emails/digest.txt',
-            'zerver/emails/followup_day1.subject',
+            'zerver/emails/followup_day1.subject.txt',
             'zerver/emails/compiled/followup_day1.html',
             'zerver/emails/followup_day1.txt',
-            'zerver/emails/followup_day2.subject',
+            'zerver/emails/followup_day2.subject.txt',
             'zerver/emails/followup_day2.txt',
             'zerver/emails/compiled/followup_day2.html',
             'zerver/emails/compiled/password_reset.html',