As a user I want to work with "corpora" not with "discourses"

[fako]

Currently you can view and add discourses. But in fact they are corpora. The interface should reflect this change and explain what a corpus is to people that don't know and are interested.

Related to this is that I think I prefer an empty "corpus" with a big + inside for a user to add their own corpus. Instead of a small button in the header.