The concept of a "personal herbarium" was a bit confused, and would, for
example vary depending on which language a user had chosen.  Due to languages
and since users can change the name of their personal herbarium, the concept
of identifying which herbarium is a user's personal herbarium really cannot
depend on the herbarium's *name*.  Thus I've added a column to herbaria
table which makes the concept explicit.  I do not do anything special to
ensure that there be only a single "personal" herbarium for a given user.
This will have to be ensured by any future code which modifies or creates
herbaria.  Might be a good idea to add a validation to Herbarium, though...