Fix the edit_note_idx_post_time_edit index in older setups to handle NULL post_time

The changes in https://github.com/metabrainz/musicbrainz-server/pull/313 should be included in the 2017 Q2 schema change release, so that older setups have this bug fix applied (fixing the sorting behavior on the /edit/notes-received page). The fix involves replacing a SQL index, which is a non-breaking change and doesn't affect any replicated parts of the database.