Implement Template.save

[philippjfr]

Partially addresses #1066

This does not yet support embedding which is what the original issue requested. It does however lay the groundwork for that.

[codecov]

Codecov Report

Merging #1224 into master will decrease coverage by 0.13%.
The diff coverage is 35.89%.

@@            Coverage Diff             @@
##           master    #1224      +/-   ##
==========================================
- Coverage   87.44%   87.30%   -0.14%     
==========================================
  Files         106      106              
  Lines       12632    12662      +30     
==========================================
+ Hits        11046    11055       +9     
- Misses       1586     1607      +21     

Impacted Files	Coverage Δ
panel/template.py	63.97% <33.33%> (-2.97%)	⬇️
panel/io/save.py	57.33% <37.03%> (-9.34%)	⬇️

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