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.

tests/homepage_caddy.bats

@@ -24,3 +24,7 @@ setup() {
     grep -q "root \* /srv/homepage" "$CADDY"
     grep -q "file_server" "$CADDY"
 }
+
+@test "the /admin/ path is rewritten to admin.html" {
+    grep -qE "@admin[[:space:]]+rewrite[[:space:]]+\*[[:space:]]+/admin.html" "$CADDY"
+}