Add support for multi-select checkbox

[doneill]

Description

Distinguish between single and multi-select choice lists

Fixes

#25

Proposed Changes

• Use checkbox for multi-select
• Reject schemas with $ref urls
  • Choice list data required to be embedded in the schema
• Bump beta version to 2.0.0-beta.8

Testing

• Run yarn test
  • Verify all tests pass
• Build and link library in React Native app

# build
npm run build

# link
yarn link

# In React Native client app, e.g. EarthRanger
yarn remove @earthranger/react-native-jsonforms-formatter
yarn link @earthranger/[email protected]