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.

shared/homepage/admin.html

@@ -4,6 +4,7 @@
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <title>frank — admin</title>
+<base href="/">
 <link rel="stylesheet" href="style.css">
 </head>
 <body>

shared/homepage/index.html

@@ -4,6 +4,7 @@
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <title>frank — services</title>
+<base href="/">
 <link rel="stylesheet" href="style.css">
 </head>
 <body>