feat: allow account creation for single-server deployments

[ayoayco]

Elk can be deployed and configured for a specific server using a nuxt runtime config singleInstance. When this is on, we can show a button to "Create an account"

Mastodon API for account creation: https://docs.joinmastodon.org/methods/accounts/#create

Mockup

In this feature we should have two buttons for single-server deployments

Mozilla Social example

I checked how mozilla.social used to do it, but seems we have to do it differently. They have their own Mozilla Accounts page, to which they redirect both "sign in" and "sign up"

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