Do not load notyf resources unless notifications are enabled

[philippjfr]

Fixes holoviz/holoviews#5469

[codecov]

Codecov Report

Merging #3958 (d242f16) into master (a5bd963) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3958   +/-   ##
=======================================
  Coverage   83.74%   83.74%           
=======================================
  Files         223      223           
  Lines       32971    32975    +4     
=======================================
+ Hits        27611    27616    +5     
+ Misses       5360     5359    -1     

Flag	Coverage Δ
ui-tests	36.95% <75.00%> (-0.01%)	⬇️
unitexamples-tests	74.98% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
panel/config.py	76.26% <100.00%> (+0.19%)	⬆️
panel/io/notifications.py	64.44% <100.00%> (+0.39%)	⬆️
panel/reactive.py	78.09% <0.00%> (+0.10%)	⬆️

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