Add FlexBox layout

[philippjfr]

Adds a FlexBox layout container which is the first ReactiveHTML based layout component added to panel. Exposes the power of CSS flex boxes in panel, which unlike our other containers responsively wrap depending on the size of the contents and the container.

[jbednar]

Pretty cool!

[codecov]

Codecov Report

Merging #2233 (db14964) into master (fbe2e2e) will decrease coverage by 0.03%.
The diff coverage is 58.53%.

@@            Coverage Diff             @@
##           master    #2233      +/-   ##
==========================================
- Coverage   83.67%   83.63%   -0.04%     
==========================================
  Files         182      183       +1     
  Lines       21869    21894      +25     
==========================================
+ Hits        18298    18311      +13     
- Misses       3571     3583      +12     

Impacted Files	Coverage Δ
panel/__init__.py	100.00% <ø> (ø)
setup.py	0.00% <ø> (ø)
panel/reactive.py	76.15% <50.00%> (-0.51%)	⬇️
panel/layout/flex.py	66.66% <66.66%> (ø)
panel/layout/__init__.py	100.00% <100.00%> (ø)
panel/io/reload.py	64.36% <0.00%> (-2.30%)	⬇️
panel/models/reactive_html.py	77.96% <0.00%> (+0.84%)	⬆️

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[MarcSkovMadsen]

Really hot and awesome 🥇