Timed text editor fixes

[jamesdools]

Is your Pull Request request related to another issue in this repository ?

https://github.com/bbc/react-transcript-editor/issues/150#issue-437707961

Describe what the PR does

• Stops the re-alignment from running during the JSON save. This caused every WrapperBlock to be created again from scratch.
• Rewrote / refactored various methods
• Pulled complex draft logic into utils
• Fixed some prop types etc.

State whether the PR is ready for review or whether it needs extra work

Worth a review now. It's difficult to measure the performance. But plan to also optimise the TranscriptEditor component and it's settings / high level state to stop those rendering also.

Additional context

For testing performance with long media, best to review via:

npm run build:storybook
npm run build:storybook:serve

This is because npm start will use the development build of React, which is a lot slower, and not a correct comparison if testing with what's on the GH pages.