Add favicon and comment

[yaojingguo]

1. If favicon is missing, Chrome and Firefox still insist on finding it.
2. Add comment clarifying the use of ReturnUri session value.

[codecov-io]

Codecov Report

Merging #146 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #146   +/-   ##
=======================================
  Coverage   56.37%   56.37%           
=======================================
  Files          11       11           
  Lines         800      800           
=======================================
  Hits          451      451           
  Misses        244      244           
  Partials      105      105           

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