New user not always required

[ScaredyCat]

Ubuntu distributions require the installed to create a non-root user as part of the installation process. As such there's no real reason to add a 'new' user during the Mastodon install that wont be used.

Making it clearer that if there's already a user on the system that you'd rather use, the user should enter that. Also make it clear that there are no detrimental effects to that account.

[techbitsio]

For local installations, I agree, so I'll add the option.

For anyone else wondering: most cloud VPS providers deploy from a template and they tend to be deployed only with root user.