Migrate to TypeScript

The project's source code is migrated to TypeScript from JavaScript. The functionality did not change.

There are some major package updates:

• Apollo 1.x to 2.x
• React 15.x to 16.x
• React Scripts 1.x to React Scripts TS 2.x

MVP

Basic functions are working (happy path)

• Log in / sign up
• Create Lean coffee session
• Change session states (topic collection / voting / discussion)
• Add topics to the session
• Count votes
• Change topic statuses on the discussion page
• Use GraphQL subscription for live updates