Ensure grid templates support overriding items

[philippjfr]

Fixes #1741

[codecov]

Codecov Report

Merging #1758 (39946a1) into master (06530a0) will increase coverage by 0.39%.
The diff coverage is 96.29%.

@@            Coverage Diff             @@
##           master    #1758      +/-   ##
==========================================
+ Coverage   84.93%   85.33%   +0.39%     
==========================================
  Files         151      151              
  Lines       17420    17861     +441     
==========================================
+ Hits        14796    15242     +446     
+ Misses       2624     2619       -5     

Impacted Files	Coverage Δ
panel/template/base.py	72.65% <50.00%> (+0.29%)	⬆️
panel/widgets/slider.py	93.95% <81.81%> (+2.25%)	⬆️
panel/layout/card.py	95.74% <100.00%> (ø)
panel/layout/grid.py	76.82% <100.00%> (+0.57%)	⬆️
panel/template/react/__init__.py	100.00% <100.00%> (ø)
panel/tests/test_server.py	100.00% <100.00%> (ø)
panel/tests/test_template.py	79.13% <100.00%> (+2.52%)	⬆️
panel/tests/widgets/test_slider.py	100.00% <100.00%> (ø)
panel/pane/alert.py	100.00% <0.00%> (ø)
panel/tests/test_docs.py	100.00% <0.00%> (ø)
... and 77 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 556c1f6...39946a1. Read the comment docs.