Add initial_state attribute to jsinput tag

Supplying an initial value for the stored state of a jsinput widget allows user to provide a start-up configuration for the external tool, which will then get updated on subsequent saves/checks as they interact with the tool.  Very useful for a generic tool (eg, a schematic editor) that we'd like to configure differently for different instances (eg, components in parts bin, available simulation modules, initial schematic).

Goal: allow course to supply a stateful interaction (with grading) that doesn't require the heavy lifting of running an XBlock through the submission and approval process.