Show the impersonator in the admin header, not the impersonated user

[davegaeddert]

The account menu in the admin's top right acts on the real, logged-in account (App Settings, Log out), but it rendered the impersonated user's avatar while impersonation was active — ImpersonateMiddleware swaps the request user, and the header reads user straight from the template context.

Changes

• AdminView.get_template_context now puts impersonator (the real user, or None) in the context, matching what the toolbar item already does.
• admin/_header.html renders the avatar for impersonator or user, so the menu's face matches the account it acts on.
• Because swapping the avatar would otherwise hide the impersonation from inside the admin, the dropdown now shows Impersonating <user> with a "Stop impersonating" item (the toolbar's existing banner is unchanged).
• Docs: a note in the ## Impersonate section of plain-admin/README.md.

Testing

• New test_admin_header_shows_the_impersonator_not_the_impersonated_user in plain-admin/tests/public/test_impersonate.py — asserts the impersonator's avatar URL is in the page, the impersonated user's is not, and the stop item is present. Verified it fails without the template change.
• The test app's User gained a get_avatar_url() so the header's avatar branch is actually exercised.
• ./scripts/test plain-admin (35 passed), ./scripts/type-check plain-admin clean, ruff check/ruff format clean. The oxlint step of ./scripts/fix couldn't download its binary in this sandbox; no JS/CSS was changed.

Note (not changed here)

pinned_slugs / get_nav_tabs still key pinned nav items off the request user, so pinned tabs come from the impersonated user's account while impersonating. Left alone as a separate question from the avatar.

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