Improve UX when registration is disabled

[e-five256]

Is your feature request related to a problem? Please describe.

Right now landing on an Mbin instance with registration disabled creates a confusing user flow as the links are just omitted. Browsing manually to /register still works, but the link is omitted on the login page, so don't believe there is a way to get to this page in the image without typing in the URL

Describe the solution you'd like

If registration is disabled, communicate that to the user in a way they can access easily (such as on the login page or restoring the link to register on the login page). Potentially do what we do on 5xx pages and link to how to find other servers as well. Similar to mastodon:

Describe alternatives you've considered

As a short-term change, perhaps the register link should always appear on the login page, even if registration is disabled, as the registration page itself shows copy to inform users that it isn't possible

Additional context

Saw a post recently in the fediverse that someone was confused on how to register on an Mbin instance, and it turned out they had landed on a developer's instance