Replace FileSystem with AssetRepository

[bartfeenstra]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 97.39583% with 5 lines in your changes missing coverage. Please review.

Project coverage is 95.74%. Comparing base (c2a2ebc) to head (65e764b).
Report is 317 commits behind head on 0.4.x.

Files with missing lines	Patch %	Lines
betty/assets.py	94.64%	2 Missing and 1 partial ⚠️
betty/tests/test_assets.py	98.38%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            0.4.x    #1613   +/-   ##
=======================================
  Coverage   95.74%   95.74%           
=======================================
  Files         198      200    +2     
  Lines       18746    18756   +10     
  Branches     3558     3558           
=======================================
+ Hits        17948    17958   +10     
  Misses        538      538           
  Partials      260      260           

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