Purge unused viewpoints

[benel]

When 100 students used Argos with LaSuli, they created far more viewpoints that what they needed.
One of them even created 250 viewpoints himself!

An easy way to purge most of those viewpoints would be:

1. to generate a view of all viewpoints having no topics,
2. to build a list of this view conforming to the bulk delete API of CouchDB,
3. to post this list with JQuery.

[benel]

I'm not sure about the way to purge corpora:

• with no viewpoint registered to the same user?
• with no related items?

[benel]

We could also add the case where Object.keys(o.topics).length < 2.