Show description for root classes

These are saved under `classDescriptors.property` on the InterMine backend. Currently not available as a web service, but we may want it in the future.

I don't think there's a suitable place to put the description in the model browser, so we might consider adding a wizard similar to the one present in the legacy webapp.

Start by reviewing the model browser and the legacy webapp wizard mentioned in the issue, then confirm how classDescriptors.property is represented by the InterMine backend. Done means root-class descriptions are available in the model browser through an agreed interface, despite the current web-service limitation.

Written by the indexing model from the issue text.