Add hide_header option for Card

[philippjfr]

Sometimes you still want to place things in cards but don't actually need a header.

[codecov]

Codecov Report

Merging #2947 (225259b) into master (336bf0e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2947   +/-   ##
=======================================
  Coverage   83.14%   83.14%           
=======================================
  Files         190      190           
  Lines       24867    24869    +2     
=======================================
+ Hits        20675    20677    +2     
  Misses       4192     4192           

Impacted Files	Coverage Δ
panel/layout/card.py	92.72% <100.00%> (+0.13%)	⬆️
panel/models/layout.py	100.00% <100.00%> (ø)

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