awebshell library: Add Web Shell URL Validator

[waterimp]

Add Web Shell URL Validator so that we can ensure quality commands are added to the database.

WebShell.validate_web_shell_url() is currently not implemented.

Unfortunately, all the code in WebShell.evaluate() and its supporting methods are not set up to do just validation, so some code refactoring is needed.

[waterimp]

This can also be used to help automatically figure out which yubnub commands are currently compatible with this system.