Allow ability to input sort field when creating Page Provider

Would like the ability to input the field to sort on instead of choosing from a dropdown. This is for fields that are defined externally or in ES mapping (for instance dc:title.lowercase).

To Do

As a user, when I go to page provider screen, under the Default sort section 

We will add an entry in the first dropdown list called 'manual field' 

This will change the dropdown to the right to a manual input field (formatting to be finalised during dev)

There will be no checks on this field, it will be contributed as is to the instance 

The value of this field will be applied as a sort 

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AC 

I can add a manual sort field to my page provider