Default profile document should include primaryTopic

[happybeing]

Although it isn't mandatory to include primaryTopic in the WebID profile, I suggest it is included to reduce the chances of breaking apps which expect it based on the examples in the spec and the fact that Solid profile generators are very likely to do so.

If not, it makes it likely that Solid apps trying to run on SAFE will need to be modified. Plume being one example so far encountered in a very small sample.