Column tests should catch missing indexes on foreign keys

[rsyring]

Postgres does not create indexes on columns with a foreign key. They have to be created manually. Our ColumnCheck for a FK usually looks something like:

ColumnCheck('space_id', fk='space_spaces.id', required=True)

My proposal would be for that format throw an exception for "Foreign key has not specified an index." The new correct options would be:

# When an index exists
ColumnCheck('space_id', fk='space_spaces.id', required=True, index='ix_categories_space_id')
# When you specifically don't want an index for some reason
ColumnCheck('space_id', fk='space_spaces.id', required=True, index=False)

This at least makes it explicit.

We could query the tables indexes and see if any index has the same column/name as the FK column as it's first column, but I think explicit/manual might be better here? Actually make the dev think about what they want to do with the indexes.