[Feature] .env Var to disable public sign up

[ReessKennedy]

Current

Currently it seems public sign up is hardcoded in.

Desired

Way to switch this off via environment variable.

Why

We may try to use this for a small team with only a few collaborators and we don't want to worry about the api allowing anyone to create an account.

Work done on this so far

• Searched around to see if there was an existing environment var that could handle this but it doesn't look like it.
• I guess the only UI element for adding is on the login page so this might require a new user-management UI letting in settings too

I could work on this and submit as a pull request, if anyone else thinks this a useful idea.