Add Row Level Security on event and occurrence table

[ninsbl]

In order to improve data security we should add Row Level Security on event and occurrence table for UPDATE and DELETE commands, so we avoid that people can damage each others data.
However, we might need a new "nofa_curator" group role for people that are eligible to quality check data that was entered.

More on RLS here:
https://www.postgresql.org/docs/9.5/static/ddl-rowsecurity.html
https://www.postgresql.org/docs/9.5/static/sql-createpolicy.html

If someone is interested in doing this feel free to assign yourself.