Add foreign keys and indices to postgresql schema.

[guidorice]

This PR modifies the PG database schema.

• Makes foreign keys explicit.
• Adds indices to support joins with foreign keys, and to support queries we have seen in production.

Known issues

• Some tables may contain so few records that the PG query planner will not actually use the indices, but at least they will be in place for future.
• Adding the constraint changesets_users_id_fk into changesets is going to cause problems: Executing this on a development db, I got an error because some user_ids are not present in the user table:

alter table changesets
add constraint changesets_users_id_fk
foreign key (user_id) references users;

The missing users issue should be resolved before changing the table create statement here.

Please beware because I have not done any integration testing and do not really have any way to see what side effects it will cause. 😄

@CloudNiner I marked this as a Draft PR, because of the user_id concern.

[guidorice]

@CloudNiner thanks for the assist- this is ready for review now.