email_log: Only import Django test client when using it.

Importing the Django test client is somewhat expensive, and we only
use it within one view function that's not used in production.  So
there's a significant startup-time performance optimization in doing
an import inside the view code.

zerver/views/email_log.py

@@ -1,7 +1,6 @@
 from django.conf import settings
 from django.http import HttpRequest, HttpResponse
 from django.shortcuts import render, redirect
-from django.test import Client
 from django.views.decorators.http import require_GET
 from django.views.decorators.csrf import csrf_exempt
 
@@ -19,7 +18,6 @@ import os
 from typing import List, Dict, Any, Optional
 import datetime
 ZULIP_PATH = os.path.join(os.path.dirname(os.path.abspath(__file__)), '../../')
-client = Client()
 
 def email_page(request: HttpRequest) -> HttpResponse:
     if request.method == 'POST':
@@ -43,6 +41,11 @@ def clear_emails(request: HttpRequest) -> HttpResponse:
 
 @require_GET
 def generate_all_emails(request: HttpRequest) -> HttpResponse:
+    # We import the Django test client inside the view function,
+    # because it isn't needed in production elsewhere, and not
+    # importing it saves ~50ms of unnecessary manage.py startup time.
+    from django.test import Client
+    client = Client()
 
     # write fake data for all variables
     registered_email = "hamlet@zulip.com"