Duplicate Users

[clicky]

Describe the bug
Currently a user can enter any name they wish, which means users can accidentally overlap.

To Reproduce

1. Start a Shared Model
2. Enter your name
3. Open this shared model from another instance of Rhino
4. Enter the same name

Expected behavior
The server should inform the user that this name is already taken and suggest an alternative name. Such as Paul -> Paul_1.

** Development Required**
A /users endpoint would need to be added to the server which returns a list of all the currently joined users.
And likely to prevent users names from getting stuck, an endpoint /signout to tell the server that a user has left.

[clicky]

Started Users section in server to resolve this potential issue