Design for importing publications

Improves design of overlay for importing publications.
You could also do:
```
Deps.autorun ->
  ImportingFile.documents.find({publicationId: $exists: true}, {fields: publicationId: 1}).forEach (file) ->
    Meteor.subscribe 'publications-by-ids', file.publicationId
```

This would make sure you sure you subscribe only to new ones and unsubscribe only to those which are not anymore in `ImportingFile`. Meteor makes sure there is no unnecessary resubscriptions between autoruns. (So if autorun subscribes to the same thing as it was subscribed before, it does not first unsubscribe and then resubscribe, but just keeps it on.)

What is more effective is a good question. Would it be better to have multiple subscriptions open and just adding to them, or one, which is resubscribed every time anything change. Maybe you should define also sort order of returned IDs, so that at least that is kept unchanging.