Addon Viewport - fix "defaultViewport" configuration

[igor-dv]

Issue: #4007

What I did

1. configureViewport was failing if "viewports" field was not defined
2. defaultViewport was not persistent with navigation

P.S. Navigating between stories will always reset to the "defaultViewport" value, IMO it's a strange behavior when manually changing the viewport and navigating to a different story.

P.P.S the branch is called addon-jest/fix-configureviewport but I meant addon-vieport/fix-configureviewport

[codecov]

Codecov Report

Merging #4683 into master will decrease coverage by 0.02%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #4683      +/-   ##
==========================================
- Coverage   35.59%   35.56%   -0.03%     
==========================================
  Files         557      557              
  Lines        6732     6731       -1     
  Branches      884      885       +1     
==========================================
- Hits         2396     2394       -2     
- Misses       3876     3877       +1     
  Partials      460      460

Impacted Files	Coverage Δ
addons/viewport/src/manager/components/Panel.js	94.82% <66.66%> (-1.79%)	⬇️

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[shilman]

LGTM!