Expose base_url in global context for templates

Since `DPASTE_BASE_URL` is a dynamic setting that can be changed when
the application is self-hosted, the templates need to have access to the
variable when rendering the templates so that they do not erroneously
display the dpaste.de url.

dpaste/context_processors.py

@@ -2,6 +2,7 @@ from django.conf import settings
 
 def dpaste_globals(request):
     return {
+        'base_url': getattr(settings, 'DPASTE_BASE_URL', 'https://dpaste.de'),
         'site_name': getattr(settings, 'DPASTE_SITE_NAME', 'dpaste.de'),
         'jquery_url': getattr(settings, 'DPASTE_JQUERY_URL',
             'https://ajax.googleapis.com/ajax/libs/jquery/1/jquery.js'),