Add project user to check for write permissions in the Rest.li deploy call

Add project user to check for write permissions in the Rest.li deploy call. Typically, a headless admin account will be used in the deploy call. This user can write to all projects. But we want to capture the fact that user "Joe" was the one that actually requested the deployment. We should check that deployments requested by Joe can only write to Joe's projects.

Note that I form the User object directly from the user name rather than getting it from the UserManager. In this case, we do not have Joe's password. The UserManager's getUser method requires the password, and providing an alternate method seems to break unwritten invariants about UserManager's method. In particular, providing a getUser method that does not take a password will require a lot of rework to LinkedIn's custom UserManager class.

This means I just check that the User has write permissions directly on the project. If the user is a part of a Group or has a Role that can write to the project, this will not be considered.

For LinkedIn's case, this will work. Generally this will probably work as well, since physical humans requesting deployments will likely be users on projects to which they can write to (rather than being part of some Group or having some Role that can write to a lot of projects). This is not ideal, but I'm not sure how else to handle this.
If the client does not include the projectUser param, won't the client be able to write to anybody's project?  Also, the client is able to spoof any user by just setting projectUser to any user, right?