Improve model for namespace

[eikek]

As described in #154 , currently our solr documents have one single property namespace denoting two different things.

For User and Group entities, the namespace porperty defines the namespace. Users and groups share a common namespace for their handles. Then a Project is defined inside a namespace - belonging either to a group or user. The Project uses for "defined inside that namespace" the same namespace property.

This means that when developing the difference must be known and taken care of implicitly. It also can make efficient searching hard to implement, if (as in #154) for some results sub-queries should be run based on only one of the meanings. Currently to resolve namespaces for projects, we resolve also the namespaces of users and groups to itself, that will result in potentially returning a lot of data from SOLR that we just ignore. It is obviously better to not ask for it in the first place.

Therefore, I think it is better to distinguish these two things more clearly. For example, by introducing another property on the project in_namespace or similar. And then remove the namespace property on projects.

The user query can be easily adopted to retain the ambiguity, but sub queries for resolving namespace details can be optimized. It will make it easier/clearer when adding more namespace-d entities later.