Make the alert type in horizon.alert translatable

Instead of just capitalizing the class name, I do a lookup in a
dictionary of human-readable (and translatable) names for the
different alert types. This also lets me use "Notice" in place of
"info", which sounds better at least to my ears.

Change-Id: I985c25a7228aee9cf1cbce222cec4bca0db05d35
Closes-bug: #1224347

horizon/static/horizon/js/horizon.messages.js

@@ -7,7 +7,13 @@ horizon.alert = function (type, message, extra_tags) {
   var template = horizon.templates.compiled_templates["#alert_message_template"],
       params = {
         "type": type,
-        "type_capitalized": horizon.utils.capitalize(type),
+        "type_display": {
+            'danger': gettext("Danger"),
+            'warning': gettext("Warning"),
+            'info': gettext("Notice"),
+            'success': gettext("Success"),
+            'error': gettext("Error")
+        }[type],
         "message": message,
         "safe": safe
       };

horizon/templates/horizon/client_side/_alert_message.html

@@ -8,7 +8,7 @@
 <div class="alert alert-block fade in alert-[[type]]">
   <a class="close" data-dismiss="alert" href="#">&times;</a>
   <p>
-    <strong>[[type_capitalized]]: </strong>
+    <strong>[[type_display]]: </strong>
     [[#safe]]
       [[[message]]]
     [[/safe]]