Tri.query

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* Moved to the simplified factory system for `from_model`

* Added shortcuts to `Variable`: url, time, datetime, date, email, decimal

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* Updated to latest major tri.form

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* Variable.choice_queryset handles the string "null" the same way as everything else

* Queries that have multi-select inputs now work

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* Report invalid operators for choice queryset as errors instead of crashing

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* Check for dates that are out of range

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* Cache form on Query

* Render global form errors

* Query.form() no longer takes a request parameter, since it was redundant anyway. This is a potential breaking change.

* Correctly dispatch attr to underlying tri.form

* Support for ajax backend. New parameter to Query: endpoint_dispatch_prefix.