Allow adjusting templates' header height and hiding it

[ahuang11]

Allows adjusting header height, and if header_height='0', the header is hidden.

Use case is for nesting templates within one another (e.g. Creating a Lumen dashboard within an app's template and hiding the Lumen's template's header)

FastListTemplate 250px

FastListTemplate 0

MaterialTemplate 250px

MaterialTemplate 0

import panel as pn

pn.extension()

template = pn.template.FastListTemplate(
    header=[
        pn.Column(
            pn.widgets.TextInput(placeholder="Header", width=250),
            pn.widgets.IntInput(placeholder="Header", width=250),
            pn.widgets.FloatInput(placeholder="Header", width=250),
        ),
        pn.widgets.TextAreaInput(placeholder="Header", width=250),
    ],
    main=["Hello", "Yolo"],
    sidebar=["Sidebar", "Testing"],
    header_height="250px",
)
template.servable()

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6a4d7f3) 84.67% compared to head (39ca703) 84.55%.
Report is 27 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6125      +/-   ##
==========================================
- Coverage   84.67%   84.55%   -0.12%     
==========================================
  Files         296      296              
  Lines       44096    44322     +226     
==========================================
+ Hits        37338    37477     +139     
- Misses       6758     6845      +87     

Flag	Coverage Δ
ui-tests	40.70% <50.00%> (+0.02%)	⬆️
unitexamples-tests	72.42% <100.00%> (-0.26%)	⬇️

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