Ensure that URL query parameters are preserved during OAuth

[philippjfr]

Fixes #3555

[codecov]

Codecov Report

Merging #3656 (b956034) into master (f4cf0ff) will decrease coverage by 0.12%.
The diff coverage is 17.77%.

@@            Coverage Diff             @@
##           master    #3656      +/-   ##
==========================================
- Coverage   82.32%   82.20%   -0.13%     
==========================================
  Files         205      205              
  Lines       27893    27934      +41     
==========================================
  Hits        22963    22963              
- Misses       4930     4971      +41     

Impacted Files	Coverage Δ
panel/auth.py	39.34% <17.77%> (-2.11%)	⬇️
panel/io/callbacks.py	74.61% <0.00%> (-3.08%)	⬇️
panel/io/reload.py	69.56% <0.00%> (-2.18%)	⬇️
panel/util.py	85.60% <0.00%> (-0.38%)	⬇️
panel/io/server.py	76.35% <0.00%> (-0.21%)	⬇️

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