Homepage (TDD slice 7): fix the /admin/ path + status.json perms

Two real bugs found on the first production deploy:
- /admin/ 404'd: file_server looked for /srv/homepage/admin/ (a dir), but the
  page is admin.html. Add '@admin rewrite * /admin.html' so the /admin/ path
  serves admin.html.
- The pages used relative refs (style.css, app.js, services.json, status.json),
  which from /admin/ resolved to /admin/... (404). Add <base href="/"> so all
  refs resolve to the site root on both pages.
- status.json was written 0600 (root); the checker now chmods it 0644 so Caddy
  can serve it regardless of its user.

dankclaw/Caddyfile

@@ -54,6 +54,7 @@ dankclaw.franksdank.cloud {
 	basicauth @admin {
 		frank REPLACE_WITH_ADMIN_HASH
 	}
+	@admin rewrite * /admin.html
 	root * /srv/homepage
 	file_server
 }