Design for importing publications

Improves design of overlay for importing publications.
Hm, not sure why you are complicating here and not simply use autorun?

```
Deps.autorun ->
  Meteor.subscribe 'publications-by-ids', _.pluck ImportingFile.documents.find({publicationId: $exists: true}, {fields: publicationId: 1}).fetch(), 'publicationId'
```

Of course you do not have to make a one-liner. :-)