Digest: Fix URIs for emoji and friends in email links.

It turns out we were using malformed URLs in the image tags
(containing just a hostname, but no http(s)!) in what we were passing
to the Django templates for our digest/, which resulted in the Django
templates treating these URLs as http.  Gmail recently cracked down on
loading images in HTTP, causing the emoji links to appear broken in
emails Zulip sends.

Fixes #3258.

zerver/lib/digest.py

@@ -121,7 +121,7 @@ def gather_new_streams(user_profile, threshold):
         new_streams = list(get_active_streams(user_profile.realm).filter(
                 invite_only=False, date_created__gt=threshold))
 
-    base_url = u"https://%s/#narrow/stream/" % (settings.EXTERNAL_HOST,)
+    base_url = u"%s/#narrow/stream/" % (user_profile.realm.uri,)
 
     streams_html = []
     streams_plain = []