Improved the first-time setup experience for new homeserver administrators:
- Account registration is disabled on the first run, except for with a new special registration token that is logged to the console.
- Other helpful information is logged to the console as well, including a giant warning if open registration is enabled.
- The default index page now says to check the console for setup instructions if no accounts have been created.
- Once the first admin account is created, an improved welcome message is sent to the admin room.

Contributed by @ginger.