fix: Unique url names for file icon

[fsbraun]

Description

ImageAdmin is a subclass of FileAdmin. get_url of file admin needs to include the admin's model name into the file icon url name.

Related resources

• URL name "filer_file_fileicon" defined twice #1421
• #...

Checklist

• I have opened this pull request against master
• I have added or modified the tests when changing logic
• I have followed the conventional commits guidelines to add meaningful information into the changelog
• I have read the contribution guidelines and I have joined #workgroup-pr-review on
  Slack to find a “pr review buddy” who is going to review my pull request.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (dbeda3a) 75.22% compared to head (6b7d1ce) 75.22%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1423   +/-   ##
=======================================
  Coverage   75.22%   75.22%           
=======================================
  Files          75       75           
  Lines        3455     3455           
  Branches      555      555           
=======================================
  Hits         2599     2599           
  Misses        691      691           
  Partials      165      165           

Files Changed	Coverage Δ
filer/admin/fileadmin.py	94.50% <ø> (ø)

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