Add license headers to all source files

LICENSE

src/config.rs

src/contestreader_yaml.rs

src/core.rs

src/db_apply_migrations.rs

src/db_conn.base.rs

src/db_conn.rs

src/db_conn_postgres.header.rs

src/db_conn_sqlite_new.header.rs

src/db_objects.rs

src/helpers.rs

src/main.rs

src/oauth_provider.rs

src/webfw_iron.rs