feat(signup): allow to block signup

[arthurdotwork]

fixes #3201.

I assumed that the flow would look like this; you start with the signup being enabled (which is the default value if the environment variable is not set), you first register to create the owner account/workspace and then, when you are done, you can now disable the registration of new users. There is still an exception; users who have a workspaceInviteHash are free to enter.

[github-actions[bot]]

	Warnings
:warning:	Changes were made to the environment variables, but not to the documentation - Please review your changes and check if a change needs to be documented!

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[FelixMalfait]

Oh yes good catch on the inviteHash, forgot to mention that!

[arthurdotwork]

Tests were failing since I had forgotten to update the service declaration. That should be OK now.

[arthurdotwork]

Hey there! 👋

Finally went through all of this GraphQL thing and was able to complete the frontend part.

https://github.com/twentyhq/twenty/assets/35616365/70edf607-a4e7-4e35-9283-a303f536dce0

[charlesBochet]

@arthureichelberger I have tested all cases and I have actually removed the override of checkUserExists in case of disabled sign up. This is preventing users to be added through inviteLink.

I feel the frontend approach is better now that we have it. So it will still be possible to list users but I feel it's OK