Stuff of a general nature, to be specific.

Add btree_gist to allow exclusion constraints with a combination of = with text
and && with tstzrange.

Remove duplicate creation of agent statuses, and convert CREATE TABLE AS to
regular INSERT since we've already created the table.

Add primary keys where appropriate.

Add a secret_missions table with timestamptz range type and exclusion
constraint preventing any agent being in the same country at the same time as
any other agent.  Also give it a couple foreign keys.