[web] Move request handling logic into cubicweb application. (closes #2200684) We improve http status handling in the process: ``application.publish`` have been renamed to ``application.handle`` to better reflect it's roles. The request object gain a status_out attribute to convey the HTTP status of the response. WSGI and etwist code have been updated. Exception gain status attribute

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if repo.system_source.dbdriver == 'postgres':

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    sql('ALTER TABLE appears ADD COLUMN weight float')

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    sql('UPDATE appears SET weight=1.0 ')