MAINT:Change ontology term identifier to bigserial

The term identifier in the ontology terms table was set as a big int, thus
adding new user-defined terms was cumbersome, with this change in place those
the term is now a bigserial.

I have also changed the ontology file in the beast, so people WILL HAVE to
reload their databases.
Do we actually need to keep this ID? It seems odd to make all deployments of Qiita have these `old_term_id`s