Add quill.js based TextEditor widget

[philippjfr]

The Ace editor is a bit heavyweight while the TextArea is a bit too simple for many use cases so we add a WYSIWYG editor which accepts and outputs HTML:

[codecov]

Codecov Report

Merging #2875 (7810f62) into master (abea3b0) will increase coverage by 0.03%.
The diff coverage is 92.10%.

@@            Coverage Diff             @@
##           master    #2875      +/-   ##
==========================================
+ Coverage   83.04%   83.08%   +0.03%     
==========================================
  Files         188      190       +2     
  Lines       24460    24524      +64     
==========================================
+ Hits        20313    20375      +62     
- Misses       4147     4149       +2     

Impacted Files	Coverage Δ
panel/config.py	58.95% <ø> (ø)
panel/widgets/ace.py	100.00% <ø> (ø)
panel/models/quill.py	86.36% <86.36%> (ø)
panel/widgets/__init__.py	100.00% <100.00%> (ø)
panel/widgets/texteditor.py	100.00% <100.00%> (ø)
panel/tests/test_param.py	99.87% <0.00%> (+<0.01%)	⬆️
panel/param.py	87.67% <0.00%> (+0.04%)	⬆️
panel/util.py	82.73% <0.00%> (+1.20%)	⬆️
panel/auth.py	40.80% <0.00%> (+1.64%)	⬆️
panel/io/reload.py	71.11% <0.00%> (+2.22%)	⬆️

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