Fix ngl state encoding

[ceesem]

Various fixes for better handling neuroglancer states.

• Fix json encoding for state server upload to make integers encoded as strings, as neuroglancer expects.
• Add version.json endpoint for a neuroglancer URL to get build info.
• Add middleauth to state service URL endpoints for Cave Explorer deployments.
• Use neuroglancer version endpoint to guess target site deployment (to use middleauth+ prefix or not) if not specified.