user interface for validating argument graphs

[tfgordon]

The Carneades engine now has a functions for validating arguments, but there is not yet any way to access these functions from the user interface.

Both single arguments and entire argument graphs can be validated. One idea would be to add "validate" buttons to argument graph and argument pages, to validate the entire argument graph or the particular argument, respectively.

Executing the validate command should produce an HTML page with a validation report, listing an problems found. The report should contain hyperlinks for navigating directly to an invalid argument.