'extrem' is misppelt throughout the schema as 'exterm'.

This issue has been migrated from [#1754](https://github.com/matrix-org/synapse/issues/1754).

---

```
./delta/35/stream_order_to_extrem.sql:CREATE TABLE stream_ordering_to_exterm (
./delta/35/stream_order_to_extrem.sql:INSERT INTO stream_ordering_to_exterm (stream_ordering, room_id, event_id)
./delta/35/stream_order_to_extrem.sql:CREATE INDEX stream_ordering_to_exterm_idx on stream_ordering_to_exterm(
./delta/35/stream_order_to_extrem.sql:CREATE INDEX stream_ordering_to_exterm_rm_idx on stream_ordering_to_exterm(
```

compare the filename to the table & index names. Please can we rename them - it's incredibly painful to have to remember other peoples' typos. (I'd also have thought 'leaf' would have been a better term than 'extremity', but I guess that ship has sailed)