querying for all entities to build the batch won't scale

Problem/Motivation

MetaTagDownloadForm does a query of all entities and then adds a batch operation for each entity.

This won't scale if there are LOTS of entities, because the array of IDs will get too big.

Instead, the form should add just one instance of the batch operation, and the callback should use a RANGE query and keep running itself multiple times until it's done all entities.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes