Exclude partial indexes when determining if a foreign key is indexed in specs

What does this MR do and why?

In spec/db/schema_spec.rb we assert that every foreign key column must have an index. However, we don't consider that the index may be a partial index and only cover a small subset of the rows in the table. This can lead to a false positive, where the spec passes but the column does not have index coverage, and cause problems such as #366615 (closed).

In order for the foreign key to function efficiently, the index must either:

• Cover the entire table (ie have no WHERE condition), or
• Be a partial index with a condition that exactly matches WHERE (<foreign_key_column> IS NOT NULL)

With these changes, indexes that don't meet one of the above conditions will cause schema_spec.rb to fail.

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