Approach for handling upgrades of the gist structure

[rufuspollock]

We need a proper pattern for doing this:

• Increase manifest_version
• Need a function that handles changing old structure into new
• Have a function per manifest_version change

[rufuspollock]

Propose that:

• unserializeProject function take care of checking the manifest and upgrading this
  • implication: should become async as may need to load data to do this (e.g. if this were an old style project with remote data)
• [optional] new loadProjectFromGist function to encapsulate loading from gist

Asides:

• Stuff like checkDatasetLoaded should become a thing of the past as all data loaded when the project loaded