Move Search Properties from the Models to the Application

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Most search properties are currently defined on the ORM model inside various onegov.* modules. I'm not yet sure this is a good idea, because we probably want different search properties for different applications.

Since we do not have different applications yet, this is currently not a pressing issue. But I think we should change this to a model where applications register models they would like to have search, alongside some kind of adapter which receives the model instance and is supposed to return the actual values for the document stored in elasticsearch.