Enhancements for Template modals

[philippjfr]

Replaces custom modal implementations with the native implementations for Fast, Bootstrap and Material and ensures they are sized correctly.

@MarcSkovMadsen I've also removed the footer that says "Made with". It's nice to attribute Panel/Fast etc. but in many cases not appropriate. I'd be open to adding a footer area and perhaps even adding that by default but having no way to disable it just won't work.

Lastly this PR replaces the "hacky" HTML pane which is used to send executable scripts with a TemplateActions ReactiveHTML component which can trigger specific (pre-defined) actions simply by updating a single variable.

[codecov]

Codecov Report

Merging #2269 (8a1f2a7) into master (7d42a99) will decrease coverage by 0.01%.
The diff coverage is 27.27%.

❗ Current head 8a1f2a7 differs from pull request most recent head 4135a0b. Consider uploading reports for the commit 4135a0b to get more accurate results

@@            Coverage Diff             @@
##           master    #2269      +/-   ##
==========================================
- Coverage   83.61%   83.60%   -0.02%     
==========================================
  Files         183      183              
  Lines       21928    21935       +7     
==========================================
+ Hits        18336    18339       +3     
- Misses       3592     3596       +4     

Impacted Files	Coverage Δ
panel/template/base.py	78.14% <0.00%> (ø)
panel/template/fast/base.py	93.18% <ø> (ø)
panel/template/material/__init__.py	88.23% <42.85%> (-11.77%)	⬇️

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