Serve /favicon.ico resembling current arachnys/img/icon_*x*.png files

One of the effects is that you don't get this Django error e-mail any more:
> ```
> [Django] ERROR: "GET /favicon.ico HTTP/1.0" 500 106
> ```

`.ico` was auto-created as following:
```
$ ( cd cabot/static/arachnys/img && convert icon_*x*.png favicon.ico && file favicon.ico )
favicon.ico: MS Windows icon resource - 2 icons, 48x48, 32 bits/pixel, 96x96, 32 bits/pixel
```

cabot/urls.py

@@ -25,12 +25,14 @@ from rest_framework.documentation import include_docs_urls
 
 from django.contrib import admin
 from django.views.generic.base import RedirectView
+from django.views.static import serve
 from django.shortcuts import redirect
 from django.contrib.auth.views import login, logout, password_reset, password_reset_done, password_reset_confirm
 admin.autodiscover()
 
 from importlib import import_module
 import logging
+import os
 
 logger = logging.getLogger(__name__)
 
@@ -55,6 +57,9 @@ urlpatterns = [
      # for the password reset views
      url('^', include('django.contrib.auth.urls')),
 
+     url(r'^(?P<path>favicon\.ico)$', serve, name='favicon',
+        kwargs={'document_root': os.path.join(settings.STATIC_ROOT, 'arachnys/img')}),
+
      url(r'^$', view=home_authentication_switcher,
         name='dashboard'),
      url(r'^subscriptions/', view=subscriptions,