Allow site administrator to manually create new accounts via UI

[HugoCortell]

Is your feature request related to a problem? Please describe.
Enabling sign-ups seems to get random internet bots to sign up, using email invites requires a functional email service, and the only mode left to create accounts after that involves database manipulation by hand, which I am not too good at.

Describe the solution you'd like
I'd like to keep sign-up disabled but be able to create accounts via the UI for new users without email-invites or anything like that. Just be able to pump out brand new newuser@local.local accounts as I wish.

As an admin, I'd really like to be able to just make accounts to give.

Describe alternatives you've considered
Biting the bullet and enabling emails.

Additional context
Vikunja supports this kind of account creation, which makes me believe it isn't a strange thing to ask for.

[HugoCortell]

From the logs, it seems that I can make a fake invite email that generates an invite link that I can grab from the logs, but personally speaking, having to open the SSH console to browse logs just to grab a link feels a little inefficient. I'd like to just be able to generate the link in the UI, then copy it from the UI. That'd be great!

[HugoCortell]

Going to test if maybe: TR_SIGNUPS_ENABLED=true TR_MAX_PROJECTS_PER_USER=0 is enough to make thing work, hopefully there's no email validation.

[HugoCortell]

Going to test if maybe: TR_SIGNUPS_ENABLED=true TR_MAX_PROJECTS_PER_USER=0 is enough to make thing work, hopefully there's no email validation.

This does not work. Damn.

[HugoCortell]

#525