Improve conflict resolution for db/schema.rb

The merge_db_schema project will resolve conflicts in the version of the schema, but it does not resolve any remaining conflicts in a particular way. This means that if there is a conflict further down the schema file, Git may sometimes fail to resolve them.

This commit vendors some of the code of the mentioned project, and changes it to use the "ours" strategy for resolving any further conflicts. This in turn should prevent the Merge Train from failing when there is a conflict in db/schema.rb.