add support for user-defined named collections

Users should be able to define and name collections. By default these would be public entries which others could rename. They would be a list of items tagged with a certain collection ID; others should be able to modify the application of the tag over time. An index/overview page for the collection should be created as well, just as for privately curated collections.

The link to the collection should be permanent, generated on creation. Whether an additional versoined permaling to a certain version of the collection is important or feasible is a separate question.