Improve template and resource management for png export

[philippjfr]

No description provided.

[codecov]

Codecov Report

Merging #2221 (3cc4bdd) into master (7c2fe60) will decrease coverage by 0.02%.
The diff coverage is 43.47%.

@@            Coverage Diff             @@
##           master    #2221      +/-   ##
==========================================
- Coverage   83.68%   83.65%   -0.03%     
==========================================
  Files         182      182              
  Lines       21859    21866       +7     
==========================================
  Hits        18293    18293              
- Misses       3566     3573       +7     

Impacted Files	Coverage Δ
panel/viewable.py	72.38% <ø> (ø)
panel/io/save.py	57.79% <43.47%> (-2.01%)	⬇️
panel/io/reload.py	64.36% <0.00%> (-2.30%)	⬇️

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