Pages: fix deletion summary routing

[agoujot]

• make the JS call the right URL
• readd a username param which shouldn't have been removed afaics
• allow slashes in the page title, which is the before-last param, and ensure the last one is non-empty, so the title doesn't include it (this fixes subpage support)
  Bug: T414768

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.69%. Comparing base (4b8eb28) to head (6dbfc20).
⚠️ Report is 237 commits behind head on main.

Additional details and impacted files

@@              Coverage Diff              @@
##               main     #612       +/-   ##
=============================================
- Coverage     71.30%   60.69%   -10.62%     
- Complexity     1275     1409      +134     
=============================================
  Files            46       46               
  Lines          3792     4320      +528     
=============================================
- Hits           2704     2622       -82     
- Misses         1088     1698      +610     

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