feat: Improve the initial setup experience

- Issue a single-use token for initial account creation
- Disable registration through other methods until the first account is made
- Print helpful instructions to the console on the first run
- Improve the welcome message sent in the admin room on first run

src/service/templates/welcome.md.j2

@@ -0,0 +1,29 @@
+## Thank you for trying out Continuwuity!
+
+Your new homeserver is ready to use! {%- if config.allow_federation %} To make sure you can federate with the rest of the Matrix network, consider checking your domain (`{{ domain }}`) with a federation tester like [this one](https://connectivity-tester.mtrnord.blog/). {%- endif %}
+
+{% if config.get_config_file_token().is_some() -%}
+Users may now create accounts normally using the configured registration token.
+{%- else if config.recaptcha_site_key.is_some() -%}
+Users may now create accounts normally after solving a CAPTCHA.
+{%- else if config.yes_i_am_very_very_sure_i_want_an_open_registration_server_prone_to_abuse -%}
+**This server has open, unrestricted registration enabled!** Anyone, including spammers, may now create an account with no further steps. If this is not desired behavior, set `yes_i_am_very_very_sure_i_want_an_open_registration_server_prone_to_abuse` to `false` in your configuration and restart the server.
+{%- else if config.allow_registration -%}
+To allow more users to register, use the `!admin token` admin commands to issue registration tokens, or set a registration token in the configuration.
+{%- else -%}
+You've disabled registration. To create more accounts, use the `!admin users create-user` admin command.
+{%- endif %}
+
+This room is your server's admin room. You can send messages starting with `!admin` in this room to perform a range of administrative actions.
+To view a list of available commands, send the following message: `!admin --help`
+
+Project chatrooms:
+> Support chatroom: https://matrix.to/#/#continuwuity:continuwuity.org
+> Update announcements: https://matrix.to/#/#announcements:continuwuity.org
+> Other chatrooms: https://matrix.to/#/#space:continuwuity.org
+>
+
+Helpful links:
+> Source code: https://forgejo.ellis.link/continuwuation/continuwuity
+> Documentation: https://continuwuity.org/
+> Report issues: https://forgejo.ellis.link/continuwuation/continuwuity/issues