Move session prefixes from handlers to Application

[philippjfr]

Moving the handling of relative paths and the base urls from the tornado handlers to the Application ensures that all server implementations inherit this handling rather than merely being supported by the Tornado server implementation.

Fixes bokeh/bokeh-fastapi#24

[codecov]

Codecov Report

Attention: Patch coverage is 93.05556% with 5 lines in your changes missing coverage. Please review.

Project coverage is 82.21%. Comparing base (81ccf03) to head (9f2cbf2).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/io/server.py	88.88%	3 Missing ⚠️
panel/io/state.py	92.30%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7318   +/-   ##
=======================================
  Coverage   82.20%   82.21%           
=======================================
  Files         337      337           
  Lines       50442    50462   +20     
=======================================
+ Hits        41464    41485   +21     
+ Misses       8978     8977    -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.