FullscreenFeedback component does not have a way to change z-index

[cupatea]

Prerequisites

• Version: "@rent_avail/feedback": "0.2.17"
• Are you running from source/master: yes
• Are you using a released build: yes
• Operating system: Mac OSX
• Bits: 64

Step to reproduce (actual behavior)

Having elements on a page with a higher z-index makes them visible over fullscreen feedback

Expected behavior

I can specify the z-index using some prop in the FullscreenFeedback component

Any message or error

N/A

Resources

Here is what a great bug report would look like:

## Prerequisites

Version: Release v3.1.0
Running from: Import using webpack
Operating system: Mac OSX
Bits: 64 bits

## Step to reproduce

 - Import checkbox from `@rentalutions/elements/checkbox`
 - Add to a React component/view
 - Run the react app
 - Notice that the `Checkbox` isn't rendering correctly which seems a encoding problem
 
### Expected behavior:

 - Import `@rentalutions/elements/checkbox`
 - Add to a React component/view
 - Run the react app
 - Check is displayed with correct encoding (e.g UTF-8)

## Any message or error

No console output
...

## Resources

No resources
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