Disable demo workspace

[kendofriendo]

@darkskygit reference to #6141 does not address this issue. Disabling sign-up does not disable the demo workspace.

Anyone with the url of the selfhosted instance is able to use the demo workspace.

Originally posted by @ylbeethoven in #5975 (comment)

[JokerQyou]

Anyone with the url of the selfhosted instance is able to use the demo workspace.

This part concerned me for a while, but it turns out guests are only getting a newly created local demo workspace, not related to the cloud-synced one. Still, admins with sign-up disabled might want the feature to turn off demo workspace for guests altogether.

[lolokraus]

Since self hosted Affine can run on a public server I would agree that admins should be ale to turn off the demo workspace for guests.