Music: refactor timeline component

[sanchitmalhotra126]

A couple refactors to the Timeline component which will be useful for some upcoming work for adding a new tracks-based programming model. Refactors include migrating styles to SCSS where possible, converting Timeline to a functional component, and splitting out the timeline events into a separate component (TimelineSampleEvents). This will allow us to easily switch between sample-based timeline elements and track-based timeline elements. This should not affect any current functionality.

Links

Testing story

Deployment strategy

Follow-up work

Privacy

Security

Caching

PR Checklist:

• Tests provide adequate coverage
• Privacy and Security impacts have been assessed
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked

[breville]

Looks good. Also tested locally, and everything appears same as before!