The `database/sql` package in the Go stdlib has two methods that can run
queries against the database driver: `Query` and `Exec`. When the query
returns rows, such as "SELECT", the package expects the use of `Query`,
and returns `*sql.Rows`. When the query does not return rows, the
package expects the use of `Exec`, which returns `sql.Result`.
This changeset corrects the "media_attachments" migration to using
`ExecContext`, as "ALTER TABLE" does not return rows.
Signed-off-by: Terin Stock <terinjokes@gmail.com>