perf: make sure all foreign keys have an index

[dargmuesli]

making a column a reference to a foreign key does not add an index to that column, so for example User.postsByAuthorId will be slow unless you've manually added an index to posts.author_id

from https://www.graphile.org/postgraphile/performance/, see https://www.graphile.org/postgraphile/postgresql-indexes/#advice---foreign-key-indexes too.

So all such columns should have an index. The index migrations could be merged into the table definitions.