Make Search content view use ES index by default

When overriding the Search content view in Studio, even if the user does not check 'Use Elasticsearch index', the associated page provider will be based on ES.

It is because the default_search page provider is by default registered too use ES and, currently, the only way to do not base it on ES is via nuxeo.conf property.

We must therefore check 'Use Elasticsearch index' by default and disable it so that users can't change the value. A helper will tell the user that the only way to do not use ES is via a nuxeo.conf property.