Add logo option for BaseTemplates

[philippjfr]

Adds ability to add logo to template directly rather than using Panel objects, which were difficult to align correctly.

[codecov]

Codecov Report

Merging #1608 into master will increase coverage by 0.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1608      +/-   ##
==========================================
+ Coverage   85.63%   85.80%   +0.16%     
==========================================
  Files         147      147              
  Lines       16488    16959     +471     
==========================================
+ Hits        14119    14551     +432     
- Misses       2369     2408      +39     

Impacted Files	Coverage Δ
panel/pane/image.py	75.48% <ø> (-4.94%)	⬇️
panel/template/base.py	72.18% <ø> (-0.45%)	⬇️
panel/pane/vtk/synchronizable_serializer.py	86.69% <0.00%> (+1.98%)	⬆️
panel/pane/vtk/vtk.py	96.32% <0.00%> (+2.13%)	⬆️

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