Alias the user creation_time column

This allows rails to set the created_at automatically, and so avoids
us from having to do so in a callback. It also hides the unusual
db column name from the rest of the app.

app/views/api/users/_user.json.jbuilder

@@ -1,7 +1,7 @@
 json.user do
   json.id user.id
   json.display_name user.display_name
-  json.account_created user.creation_time.xmlschema
+  json.account_created user.created_at.xmlschema
   json.description user.description if user.description
 
   if current_user && current_user == user && can?(:details, User)

app/views/api/users/_user.xml.builder

@@ -1,6 +1,6 @@
 xml.tag! "user", :id => user.id,
                  :display_name => user.display_name,
-                 :account_created => user.creation_time.xmlschema do
+                 :account_created => user.created_at.xmlschema do
   xml.tag! "description", user.description if user.description
   if current_user && current_user == user && can?(:details, User)
     xml.tag! "contributor-terms", :agreed => user.terms_agreed.present?,