Fix Card Header in Fast Templates

[MarcSkovMadsen]

Fixes #2379 as well as unreported height and margin issue discovered during testing.

import panel as pn

d = pn.Card(pn.widgets.FloatSlider(name='Slider'),'#Hola')
e = pn.Card(pn.widgets.FloatSlider(name='Slider'),'#Hola', header="Text")
f = pn.Card(pn.widgets.FloatSlider(name='Slider'),'#Hola', header="# Text")

tmpl = pn.template.FastListTemplate(
    theme=pn.template.DarkTheme,
    main=[d,e,f]
)
tmpl.servable()

[codecov]

Codecov Report

Merging #2437 (75f92e5) into master (040b887) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2437   +/-   ##
=======================================
  Coverage   83.45%   83.46%           
=======================================
  Files         185      183    -2     
  Lines       22680    22679    -1     
=======================================
  Hits        18928    18928           
+ Misses       3752     3751    -1     

Impacted Files	Coverage Δ
panel/tests/pane/test_media.py	96.42% <0.00%> (-3.58%)	⬇️
panel/widgets/tables.py	83.56% <0.00%> (-0.22%)	⬇️
panel/pane/__init__.py	100.00% <0.00%> (ø)
panel/models/tabulator.py	100.00% <0.00%> (ø)
panel/tests/pane/test_ace.py
panel/pane/ace.py
panel/reactive.py	75.00% <0.00%> (+0.03%)	⬆️
panel/tests/widgets/test_tables.py	99.29% <0.00%> (+0.03%)	⬆️
panel/tests/pane/test_plotly.py	98.73% <0.00%> (+0.20%)	⬆️
panel/pane/plotly.py	92.75% <0.00%> (+1.32%)	⬆️
... and 2 more

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