Changes dependency on ASP.NET MVC to newer version (should have been done long ago!)
Adds PreventDoubleAction attribute to allow prevention of duplicate form submits on the server side.  See https://trello.com/card/add-action-filter-to-be-able-to-prevent-a-form-submitting-twice-on-the-server-side/4f80dfdad096c0a82e10b37f/51
Adds extension method "IsValidEditableFieldsOnly" to ModelState to be called instead of "ModelState.IsValid".  See https://trello.com/card/create-server-side-condition-evaluator-and-validate-helper-method-to-determine-if-something-s-validation-should-be-ignored-based-on-whether-it-was-editable/4f80dfdad096c0a82e10b37f/41