Add a sensible z-index to app-drawer to ensure it being on top of the contents of page components

[lozandier]

Summary of Change

• Adds z-index: 1 to fix an issue of the children of a page component end up being over app-drawer—especially if contain: content is used on any of the children.

• Ensures expectations that the z-index of app-drawer is by default higher than the contents of main within my-app.

Actions taken by change

• [x] Add z-index: 1 to app-drawer in shared

Questions to consider during review

• I'm running on empty, but I want to say in hindsight that app-drawer selector change should be in my-app.js render function.

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[lozandier]

@frankiefu Changes done; think the review comments 'disappeared' when I interactive rebased.