Add support for explicit OAuth redirect URI

[philippjfr]

Sometimes we want to explicitly provide a OAuth redirect URI.

[codecov]

Codecov Report

Merging #1468 into master will decrease coverage by 0.05%.
The diff coverage is 20.00%.

@@            Coverage Diff             @@
##           master    #1468      +/-   ##
==========================================
- Coverage   86.50%   86.44%   -0.06%     
==========================================
  Files         138      138              
  Lines       15347    15361      +14     
==========================================
+ Hits        13276    13279       +3     
- Misses       2071     2082      +11     

Impacted Files	Coverage Δ
panel/auth.py	38.23% <0.00%> (-0.38%)	⬇️
panel/command/serve.py	17.64% <0.00%> (-1.11%)	⬇️
panel/config.py	45.35% <37.50%> (-0.25%)	⬇️

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