Disable directory listing and update ignored files

[maciaszczykm]

Previously adding /assets/ at the end of the URL resulted in:

After the change:

Closes #3899.

[codecov]

Codecov Report

Merging #6326 (36dba7c) into master (94a5fd0) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #6326      +/-   ##
==========================================
- Coverage   42.53%   42.53%   -0.01%     
==========================================
  Files         217      217              
  Lines       10621    10624       +3     
  Branches      144      144              
==========================================
+ Hits         4518     4519       +1     
- Misses       5837     5839       +2     
  Partials      266      266              

[floreks]

/lgtm

[k8s-ci-robot]

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