* Add bookmark ordering

Bookmark ordering is applied to bookmark fetches order-by clauses.
To save ordering send a sequence of `[{type, item_id}]` values.
There are no foreign keys to the `*_bookmark` tables and
bookmark ordering handle extra or missing rows gracefully by ignoring
them or falling back to order by created_at as a secondary sort. Pruning
is done on save.

* Fix missing docstring

* Fix refer order

* Use varchar for type because h2 is unhappy with text index

* Cast untyped literal in union for h2

* Use hx/literal thanks @dpsutton