Various extensions and fixes to constraints and template/application admin site management

* ConstraintExpressionFilter for Application and ApplicationEntry custom admin filters
  These demonstrate the correctness of Constraint and ConstraintExpression models for
  selecting rows described by constraints and expressions
* Added submit and unsubmit actions for Applications
* 'answer' fields of ApplicationEntry and Constraint are polymorphic
* ConstraintExpression converts expression to descriptive string for human-readability
* Added create application, publish and unpublish actions for Templates
* minor bug fix in WhenPublishedThrow
* added useful syntaxes (Q, FK, Descriptive)
* added model validation to ConstraintExpression
* publication status illegal state transitions are ValidationErrors rather than general Exception
* bug fix in ApplicationForm for saving entries when commit = False
* Initial ApplicationEntryForm added (usage commented out)