(Bug 48941) Try #2: Normalize ext. attributes in ExtensionHandler

* Commit 2001f875 attempted to normalize extension attributes in
  the tokenizer.  However, this turned out to buggy since templated
  attribute values may not be fully expanded strings yet.

  en:Richter's_scale crashed.

* Moved the normalization code to extension handler, and only on
  native extension attributes (since PHP preprocessor will deal with
  the cases it handles).

* This fix still cannot handle template attribute values for extensions
  since they are not fully expanded yet at the time the native handler
  is invoked.  See FIXME in ext.core.ExtensionHandler alongwith an idea
  of how we might be able to deal with this.

  So, <ref name='  {{echo| foo}} ' /> will not normalize to
  <ref name='foo' /> at this time.  We can special case the fix for
  ref and references in ext.Cite.js, but if we can find a generic
  solution, that would be btter.

* en:Richter's_scale no longer crashes now.

Change-Id: I7e96fb6d2af444ac2561999de4b01a98a4f29275