Add default templates

[philippjfr]

This PR will add a number of default Templates which allow populating three main areas:

• Header
• Sidebar
• Main area

The templates are built on a choice of different CSS frameworks including:

• Material Web Components
• Bootstrap

Material Web Components

Bootstrap

[MarcSkovMadsen]

Going to be nice.

You will get requesst from some people regarding a footer and a title_icon. I guess. I would be needing the title_icon but not the footer personally.

[jbednar]

What about a right sidebar? Is there a big cost to having empty Divs when they aren't used?

[codecov]

Codecov Report

Merging #1277 into master will increase coverage by 0.11%.
The diff coverage is 65.26%.

@@            Coverage Diff             @@
##           master    #1277      +/-   ##
==========================================
+ Coverage   87.69%   87.80%   +0.11%     
==========================================
  Files         126      130       +4     
  Lines       13590    14076     +486     
==========================================
+ Hits        11918    12360     +442     
- Misses       1672     1716      +44     

Impacted Files	Coverage Δ
panel/viewable.py	68.80% <ø> (+0.45%)	⬆️
panel/template/base.py	56.16% <51.37%> (ø)
panel/template/theme.py	72.22% <72.22%> (ø)
panel/io/server.py	37.65% <100.00%> (ø)
panel/layout/card.py	95.74% <100.00%> (+0.09%)	⬆️
panel/template/__init__.py	100.00% <100.00%> (ø)
panel/template/bootstrap/__init__.py	100.00% <100.00%> (ø)
panel/template/material/__init__.py	100.00% <100.00%> (ø)
panel/tests/test_param.py	99.77% <0.00%> (-0.08%)	⬇️
... and 5 more

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