Add option to create user with Site

[Dave Shoreman]

This could either be handled by the project type, or should have a checkbox to enable creation of a user for a project being created.

If it's handled by the project type, we'll need to figure out which types it should be enabled for. Plain text and redirect types won't need it, but what about PHP/Laravel? Probably not in most cases.

Binary projects (such as a docker container or IRC bot) could benefit from a new dedicated project type.