Refactor email sending functions to allow customization

For example, if we need to use a custom From: address,
we can now do:

    m = make_message("template-name", recipient, ctx)
    m.from_email = "...."  # customize here
    send(m)

hc/lib/emails.py

@@ -20,7 +20,7 @@ class EmailThread(Thread):
                 # Make sure each retry creates a new connection:
                 self.message.connection = None
                 self.message.send()
-                # No exception--great, return from the retry loop
+                # No exception--great! Return from the retry loop
                 return
             except smtplib.SMTPServerDisconnected as e:
                 if attempt + 1 == self.MAX_TRIES:
@@ -32,7 +32,7 @@ class EmailThread(Thread):
                 time.sleep(1)
 
 
-def send(name, to, ctx, headers={}, from_email=None):
+def make_message(name, to, ctx, headers={}):
     ctx["SITE_ROOT"] = settings.SITE_ROOT
 
     subject = render("emails/%s-subject.html" % name, ctx).strip()
@@ -41,11 +41,13 @@ def send(name, to, ctx, headers={}, from_email=None):
 
     msg = EmailMultiAlternatives(subject, body, to=(to,), headers=headers)
     msg.attach_alternative(html, "text/html")
-    if from_email:
-        msg.from_email = from_email
 
+    return msg
+
+
+def send(msg, block=False):
     t = EmailThread(msg)
-    if hasattr(settings, "BLOCKING_EMAILS"):
+    if block or hasattr(settings, "BLOCKING_EMAILS"):
         # In tests, we send emails synchronously
         # so we can inspect the outgoing messages
         t.run()
@@ -56,36 +58,38 @@ def send(name, to, ctx, headers={}, from_email=None):
 
 
 def login(to, ctx):
-    send("login", to, ctx)
+    send(make_message("login", to, ctx))
 
 
 def transfer_request(to, ctx):
-    send("transfer-request", to, ctx)
+    send(make_message("transfer-request", to, ctx))
 
 
 def alert(to, ctx, headers={}):
-    send("alert", to, ctx, headers=headers)
+    send(make_message("alert", to, ctx, headers=headers))
 
 
 def verify_email(to, ctx):
-    send("verify-email", to, ctx)
+    send(make_message("verify-email", to, ctx))
 
 
 def report(to, ctx, headers={}):
-    send("report", to, ctx, headers=headers)
+    m = make_message("report", to, ctx, headers=headers)
+    send(m, block=True)
 
 
 def deletion_notice(to, ctx, headers={}):
-    send("deletion-notice", to, ctx, headers=headers)
+    m = make_message("deletion-notice", to, ctx, headers=headers)
+    send(m, block=True)
 
 
 def sms_limit(to, ctx):
-    send("sms-limit", to, ctx)
+    send(make_message("sms-limit", to, ctx))
 
 
 def call_limit(to, ctx):
-    send("phone-call-limit", to, ctx)
+    send(make_message("phone-call-limit", to, ctx))
 
 
 def sudo_code(to, ctx):
-    send("sudo-code", to, ctx)
+    send(make_message("sudo-code", to, ctx))