Do not suggest adding a user to the project who is already in the project

[slaFFik]

When I'm on the Project editing screen, there is a "Add user" section.
When I click inside of it - I see the current user (myself) as well, who is already in this project (which is clearly visible below in the table of project users.

Selecting that user results in an error:

Users that are already in the project should not appear in this dropdown:

[RichardAnderson]

Happy to pick this one up @saeedvaziry