Self-serve account deletion

[ArtOfCode-]

Adds self-serve account deletion.

Closes #925.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.70%. Comparing base (7faaf4d) to head (42a7b9c).
Report is 20 commits behind head on develop.

Additional details and impacted files

Components	Coverage Δ
controllers	65.38% <100.00%> (+0.38%)	⬆️
helpers	70.82% <100.00%> (+0.03%)	⬆️
jobs	48.57% <ø> (ø)
models	85.17% <ø> (ø)

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[ArtOfCode-]

Ready for review, but let's hold off this one until #1647 is merged.

[cellio]

We don't allow admins or mods to self-delete (good), but the check is at the end. Instead of sending the user through the sudo path (password, then type user name, then get the error), can we check when the person clicks the button?

Other than that, looks great! I tested all the special statuses (local/global mod/admin), and confirmed that staff alone doesn't block deletion. The deletions show up in the audit log as expected, and that email address/password no longer works to sign in. A self-deleted user can sign up again with the same email address and gets a new account.

[ArtOfCode-]

Good catch - I meant to just not show the button for special users, perhaps with a caption instead, but forgot to add that.

[cellio]

Grayed out with an explanation would be consistent with other buttons that are selectively not available. But any way you want to handle it (at the beginning) is fine.

[cellio]

LGTM. Defer merge until after 1647 merges.

[Oaphi]

LGTM code-wise (have done some manual testing too, but not beyond getting to the sudo screen).
Left a bunch of minor suggestions (mostly upkeep-related) as a separate PR with this one as a target