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
Merge "Make the alert type in horizon.alert translatable"