End OIDC sessions upstream on signout

[jraddaoui]

• Follow signout redirect/callback flow to end sessions upstream.
• Change environment configuration to set a base URL and generate the signin and signout redirect URLs from it.

Refs #957.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 53.06%. Comparing base (27b772a) to head (6edaed7). Report is 39 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #993 +/- ## ========================================== - Coverage 53.13% 53.06% -0.08% ========================================== Files 101 101 Lines 5661 5661 ========================================== - Hits 3008 3004 -4 - Misses 2406 2408 +2 - Partials 247 249 +2 ```

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