Allow guest users to browse public projects in the VRE

[jgonggrijp]

A visitor should be able to see public projects, including their collections, records and annotations, without having to authenticate themselves.

[jgonggrijp]

This is sort-of a duplicate of #95. I'm leaving this one open because it is a more up-to-date description of what should and shouldn't be possible, together with #321 and #322.

[jgonggrijp]

The description originally contained a statement that searching the catalogs should be disabled. I removed this because it was based on a misunderstanding.