Mthpower-taal

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Bugfix for non-ascii `context` or `message_id`

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Allow `base_query` to be overriden in `_normalised_translations`.

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* Allow `language` to be a callable for more dynamic behaviour.
* Enable more customisable strategy for handling missing translations, adding
an option to return a sentinel value in addition to the existing options of
`None` or debug output.
* Bugfix for `change_instance_type` when updating values to be `None`

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Updates for kaiso 0.30 compatibility (dropping legacy index support)

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Bugfix for re-saving kaiso instances loaded from the db without changing some
translatable attributes.

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Bump sqlalchemy requirement to 0.9. The signature of the `after_bulk_update`
event has changed, and the legacy compat wrapper seems broken as of sqla 0.9.4.