Allow controlling initial Template sidebar state

[philippjfr]

Add initial_sidebar_state option to templates to allow controlling whether sidebar initializes as collapsed or expanded.

[codecov]

Codecov Report

Merging #5033 (60518d4) into main (480e6b0) will increase coverage by 10.12%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##             main    #5033       +/-   ##
===========================================
+ Coverage   73.40%   83.52%   +10.12%     
===========================================
  Files         271      271               
  Lines       38361    38667      +306     
===========================================
+ Hits        28157    32295     +4138     
+ Misses      10204     6372     -3832     

Flag	Coverage Δ
ui-tests	40.67% <80.00%> (?)
unitexamples-tests	73.51% <80.00%> (+0.11%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
panel/template/base.py	78.79% <100.00%> (+0.08%)	⬆️
panel/template/slides/__init__.py	100.00% <100.00%> (ø)
panel/tests/ui/widgets/test_sliders.py	100.00% <100.00%> (+84.57%)	⬆️

... and 70 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[maximlt]

Other than some minor comments, did you not implement GoldenTemplate intentionally?