Info add-on: update z-index of container

[kohlmannj]

Issue: #1263

What I did

I updated the info addon's fixed-position container to use a z-index of 99999, thereby allowing it to appear above most other content that may appear in the preview iframe.

How to test

With stories that use the Info addon:

• Verify that the info content appears above all other preview iframe content when clicking on the "Show Info" tab
• Verify that the blue "X" tab still appears above the info content when the info content is being shown

[codecov]

Codecov Report

Merging #1264 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1264   +/-   ##
=======================================
  Coverage   13.71%   13.71%           
=======================================
  Files         207      207           
  Lines        4646     4646           
  Branches      509      496   -13     
=======================================
  Hits          637      637           
- Misses       3568     3589   +21     
+ Partials      441      420   -21

Impacted Files	Coverage Δ
addons/info/src/components/Story.js	0% <ø> (ø)	⬆️
addons/knobs/src/components/PropForm.js	8.51% <0%> (ø)	⬆️
...rc/modules/ui/components/left_panel/text_filter.js	33.33% <0%> (ø)	⬆️
app/react/src/client/preview/error_display.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/api/configs/init_api.js	40.47% <0%> (ø)	⬆️
addons/info/src/components/Props.js	0% <0%> (ø)	⬆️
addons/knobs/src/components/types/Object.js	5.81% <0%> (ø)	⬆️
...react-native/src/manager/components/PreviewHelp.js	0% <0%> (ø)	⬆️
...codemod/src/transforms/update-organisation-name.js	40.62% <0%> (ø)	⬆️
addons/knobs/src/components/types/Number.js	8.06% <0%> (ø)	⬆️
... and 12 more

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[danielduan]

I guess if you have a z-index set in your component, it could appear above the info panel.

Info will probably get refactored into its own panel but this will fix the outstanding issue for now.

[kohlmannj]

@danielduan agreed, this is just supposed to be a short-term fix. Thanks!