add a dry-run option to the entity pull form

Problem/Motivation

It would be useful to have a dry run option which shows you what would be imported, but doesn't save anything.

Steps to reproduce

Proposed resolution

I can see two ways to do this:

a. Big hammer approach -- replace ALL the entity storage handlers on submit with dummy versions

b. pass a value in the import context, and expect all processors to respect it

We also need a way to collect what's been imported and output the list.

Remaining tasks

User interface changes

API changes

Data model changes