Allow full reset of dashboard builder layout

[philippjfr]

Previously the dashboard builder UI supported resets but only in a very limited way. Specifically resetting would happen live in the frontend and reset the order, unhide hidden components, reset the width to 100% but leave the height unchanged because the original height was effectively not recoverable. This PR instead forces a reload of the page and then tells the backend not to send the layout data, which means everything resets to what it was originally.

Fixes #6600

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 82.85%. Comparing base (9bfff01) to head (450519a).
Report is 6 commits behind head on main.

Files	Patch %	Lines
panel/io/handlers.py	7.69%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6625      +/-   ##
==========================================
- Coverage   82.96%   82.85%   -0.12%     
==========================================
  Files         313      313              
  Lines       46090    46134      +44     
==========================================
- Hits        38239    38224      -15     
- Misses       7851     7910      +59     

Flag	Coverage Δ
ui-tests	40.00% <25.00%> (-0.03%)	⬇️
unitexamples-tests	71.13% <6.25%> (-0.12%)	⬇️

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