Add 'has bug ever had enhancement severity' as a feature (#247)

bugbug/bug_features.py

@@ -287,6 +287,16 @@ class is_reporter_a_developer(object):
         return bug_reporter()(bug).strip() in author_ids
 
 
+class had_severity_enhancement(object):
+    def __call__(self, bug, **kwargs):
+        for history in bug['history']:
+            for change in history['changes']:
+                if change['field_name'] == 'severity' and change['added'] == 'enhancement':
+                    return True
+
+        return False
+
+
 def cleanup_url(text):
     text = re.sub(r'http[s]?://(hg.mozilla|searchfox|dxr.mozilla)\S+', '__CODE_REFERENCE_URL__', text)
     return re.sub(r'http\S+', '__URL__', text)