Resolve "Release v1.2.0"

[1.2.0] - 2021-07-22

Added

• Provide a service worker file in public repository
• Add a callback to receive notifications when a crdt is updated

Changed

• Separate the timeout into two timeout: pushing/polling
• Remove frequent pooling but keep a backup call when it has not received a new version for a while
• Set push timeout only when there has been a write

Fixed

• Fix mdeditor new version types

Closes #44 (closed)