Clarify use of project ID for cloud servers

[SheepTester]

https://scratch.mit.edu/users/Sheep_maker/#comments-149358539

Maybe if the cloud server option is selected, a copy of the project ID can be shown if the project source isn't already from ID*. Can add a note saying that it's used to tell the server which project it's for, and projects with the same ID will share the same cloud variables

*Actually it might be helpful for this to be overrideable even if uploading by ID, but perhaps changing the project source ID will automatically change this project ID as well (unless it had been set to a different value)