Fix bundling of template resources

[philippjfr]

Fixes two issues with bundling:

1. Bundle resources shared by multiple Templates into baseclass bundle directory
2. Ensure template bundle directories are consistently lowercased

Fixes #2074

[codecov]

Codecov Report

Merging #2076 (5d084d4) into master (650c9e7) will increase coverage by 0.00%.
The diff coverage is 53.57%.

@@           Coverage Diff           @@
##           master    #2076   +/-   ##
=======================================
  Coverage   84.40%   84.41%           
=======================================
  Files         178      179    +1     
  Lines       20646    20723   +77     
=======================================
+ Hits        17427    17494   +67     
- Misses       3219     3229   +10     

Impacted Files	Coverage Δ
panel/compiler.py	7.17% <0.00%> (-0.35%)	⬇️
panel/template/base.py	80.33% <100.00%> (+0.47%)	⬆️
panel/tests/pane/test_perspective.py	100.00% <0.00%> (ø)
panel/pane/perspective.py	91.80% <0.00%> (+2.51%)	⬆️

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