Add support for percentage threshold for pages completion criteria

[bjester]

Summary

• Adds support for a percentage between 1-100%, inclusive, for the pages completion criteria
• Breaks out the mastery criteria into separate schema for reuse

[sairina]

Now that mastery and completion criteria are separated, will there need to be any updates I need to make to imports in Studio's frontend to address that?

[rtibbles]

Could you also increment the version in setup.py and rerun the build process, and commit the resulting changes in package.json too? Then we can just tag and release straight away.