Use a signal to set locale cookie after login

Effectively undoes commit 0f8018e

The approach of signals is required because of social login. See issue
#171
for details as to why.

cjworkbench/__init__.py

@@ -1 +1,2 @@
-
+# this line required to get ServerConfig.ready() to run
+default_app_config = "cjworkbench.apps.ServerConfig"

cjworkbench/apps.py

@@ -6,4 +6,4 @@ class ServerConfig(AppConfig):
     name = "cjworkbench"
 
     def ready(self):
-        pass
+        import cjworkbench.signals

cjworkbench/signals.py

@@ -0,0 +1,8 @@
+from allauth.account.signals import user_logged_in
+from django.dispatch import receiver
+from cjworkbench.i18n import set_language_cookie
+
+
+@receiver(user_logged_in)
+def set_locale_cookie_after_login(sender, *, response, user, **kwargs):
+    set_language_cookie(response, user.user_profile.locale_id)

cjworkbench/urls.py

@@ -18,12 +18,10 @@
 from django.contrib import admin
 from django.views.generic.base import RedirectView
 from cjworkbench.i18n.views import set_locale
-from cjworkbench.views.login import LoginView
 
 urlpatterns = [
     url(r"^admin/?", admin.site.urls),
     url(r"^account/signup/$", SignupView.as_view(), name="account_signup"),
-    url(r"^account/login/$", LoginView.as_view(), name="account_login"),
     url(
         r"^xyzzy/signup/$",
         RedirectView.as_view(url="/account/signup/", permanent=True),