Conflict-free schema version handling

This is a follow-up to #212051 (closed) to further reduce merge conflicts caused by the schema version information in db/structure.sql.

This targets the list of migrations that have been executed to reach the currently present database schema. Essentially the information in schema_migrations which is populated at the end of db/structure.sql.

The conflicts originate from appending to a sorted list of schema versions. The proposal here is to do the following instead:

1. When a migration executes, it touches a file in db/schema_migrations/$schema_version (or whatever directory), one file per migration
2. db/structure.sql contains the following to populate the table schema_migrations:

INSERT INTO schema_migrations (version) SELECT pg_ls_dir('db/schema_migrations');

This essentially removes the append-only style and leads to no conflicts anymore.