Assorted fixes for the Code Golf's leaderboard

[Oaphi]

This PR fixes several issues with the dashboard:

1. Answers from deleted users now show up as expected:
   
2. The leaderboard no longer shows undefined instead of username / language if either can't be determined (I chose "N/A" but not married to it - deferring to @trichoplax's judgment call):
   
3. The leaderboard now always renders in the correct place (before the number of answers header) - see the issue for details

closes #1911

As a side-effect of these changes, we now have two more hooks that userscripts / community assets can rely on: js-deleted-post and js-answers-header (neither of these is likely to go away, so let's make sure layout changes don't screw up with user-built stuff).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.56%. Comparing base (a950c26) to head (0d96cc3).
⚠️ Report is 5 commits behind head on develop.

Additional details and impacted files

Components	Coverage Δ
controllers	74.66% <ø> (ø)
helpers	84.91% <ø> (-0.49%)	⬇️
jobs	79.24% <ø> (ø)
models	90.01% <ø> (ø)
tasks	61.11% <ø> (ø)

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[trichoplax]

The layout problem I mentioned in my original review turned out to be my error (I forgot to include the community CSS file in the head file). All working perfectly now.

Thanks very much for helping me spot this in Discord.