Unify huddles and personals into private messages on the send path

Personals are now just private messages between two people (which
sometimes manifests as a private message with one recipient).  The
new message type on the send path is 'private'.  Note that the receive
path still has 'personal' and 'huddle' message types.

(imported from commit 97a438ef5c0b3db4eb3e6db674ea38a081265dd3)

examples/curl-examples

@@ -4,7 +4,7 @@
 curl https://humbughq.com/api/v1/send_message \
     -d "api-key=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx" \
     -d "email=tabbott@humbughq.com" \
-    -d "type=personal" -d "content=test" \
+    -d "type=private" -d "content=test" \
     -d "recipient=tabbott@humbughq.com"
 
 curl https://humbughq.com/api/v1/get_messages \