improve screenshot-test performance

[adrianjost]

Description

• Cherrypick from Feature/screenshot test crop screenshots to content #524
• Moved the performance improvements into this PR so we can merge it earlier :D
• parallize screenshot test execution. This also required renaming all screenshot files 😐 That's why there are so many files changed.
• ~7min => ~3min
• mocked all external images with base64 or repo internal Images

[hpi-schul-clud-bot]

❤️ I have deployed this pull-request for you:

Docs: http://docs.644.nuxt.schul-cloud.surge.sh
Storybook: http://stories.644.nuxt.schul-cloud.surge.sh

[codecov]

Codecov Report

Merging #644 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #644   +/-   ##
========================================
  Coverage    84.62%   84.62%           
========================================
  Files           77       77           
  Lines          813      813           
  Branches       131      131           
========================================
  Hits           688      688           
  Misses         122      122           
  Partials         3        3

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