dependencies: Upgrade to Django 2.2.10.

Django 2.2.x is the next LTS release after Django 1.11.x; I expect
we'll be on it for a while, as Django 3.x won't have an LTS release
series out for a while.

Because of upstream API changes in Django, this commit includes
several changes beyond requirements and:

* urls: django.urls.resolvers.RegexURLPattern has been replaced by
  django.urls.resolvers.URLPattern; affects OpenAPI code and related
  features which re-parse Django's internals.
  https://code.djangoproject.com/ticket/28593
* test_runner: Change number to suffix. Django changed the name in this
  ticket: https://code.djangoproject.com/ticket/28578
* Delete now-unnecessary SameSite cookie code (it's now the default).
* forms: urlsafe_base64_encode returns string in Django 2.2.
  https://docs.djangoproject.com/en/2.2/ref/utils/#django.utils.http.urlsafe_base64_encode
* upload: Django's File.size property replaces _get_size().
  https://docs.djangoproject.com/en/2.2/_modules/django/core/files/base/
* process_queue: Migrate to new autoreload API.
* test_messages: Add an extra query caused by .refresh_from_db() losing
  the .select_related() on the Realm object.
* session: Sync SessionHostDomainMiddleware with Django 2.2.

There's a lot more we can do to take advantage of the new release;
this is tracked in #11341.

Many changes by Tim Abbott, Umair Waheed, and Mateusz Mandera squashed
are squashed into this commit.

Fixes #10835.

zerver/lib/integrations.py

@@ -3,7 +3,7 @@ import os
 from typing import Dict, List, Optional, Any, Tuple
 from django.conf.urls import url
 from django.contrib.staticfiles.storage import staticfiles_storage
-from django.urls.resolvers import LocaleRegexProvider
+from django.urls.resolvers import RegexPattern
 from django.utils.module_loading import import_string
 from django.utils.translation import ugettext as _
 from zerver.lib.storage import static_path
@@ -178,7 +178,7 @@ class WebhookIntegration(Integration):
         self.doc = doc
 
     @property
-    def url_object(self) -> LocaleRegexProvider:
+    def url_object(self) -> RegexPattern:
         return url(self.url, self.function)
 
 class HubotIntegration(Integration):