Add 'orchestra' to z-index stage metaphor

# Update Stage Metaphor with 'Orchestra'

Status: **Opened for visibility**
Reviewers: @jeffkamo @ry5n 

## Changes
- Change what was previously `$frontrow-layer` to be `$orchestra-layer`
- Change what was previously `$overlay-layer` to be `$frontrow-layer`

## Notes
- Suggestion from @noahadams to help solidify the stage metaphor.