Add function to enforce X-Forwarded-User matches process owner

[hansen-m]

This adds a simple check to ensure the Open onDemand proxy user sent with X-Forwarded-User matches the user id of the process based on the $USER variable. In an Open onDemand HPC context, the application is then only accessible to the user who started the job and other users see a forbidden error. With this, the Open onDemand app itself doesn't need to do password or token generation.

One concern may be if in some environments if the local user name doesn't match the remote Open onDemand user name but unsure how common this configuration is used.