Multiple index-fields files

Index Fields
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Since subtemplates are stored in the metadata table and treated as metadata, it can be useful to index different sub-templates in different ways in order to add different data to the index.  

This does 2 things:

 1. Moves index-fields.xsl to index-fields/default.xsl and language-index-fields.xsl to index-fields/language-default.xsl.
 2. Changes SearchManager so that when it looks for an index-fields and language-index-files it first checks for: index-fields/<root>.xsl and index-fields/language-<root>.xsl.  If these are not found then it falls back to index-fields/default.xsl and index-fields/language-default.xsl.

iso19139's subtemplate indexing is unchanged so current behaviour is exactly the same as previously but schema-plugins can take advantage of this new option.


Extra
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I also added an  `extra` field to Metadata table and the index for storage arbitrary data.  The purpose is simply to allow arbitrary data to be stored and searched on.  This is primarily for geonetwork based applications so they don't need to change the model to add an extra search option.