Add 'orchestra' to z-index stage metaphor

# Update Stage Metaphor with 'Orchestra'

Status: **Opened for visibility**
Reviewers: **(GitHub @<name>s of all reviewers)**
JIRA: [{TICKET}](https://mobify.atlassian.net/browse/{TICKET})

## Changes
- Changed what was previously `$frontrow-layer` to be `$orchestra-layer`
- Changed what was previously `$overlay-layer` to be `$frontrow-layer`

## Notes
- Suggestion from @noahadams to help solidify the stage metaphor.