Add user impersonation in CLI via env

This adds the capability to impersonate another user (setting username
and full_name) via the CLI. It accomplishes this by reading the
environment variables NIPAP_IMPERSONATE_USERNAME and
NIPAP_IMPERSONATE_FULL_NAME. Naturally it only works when a user it
trusted.
Merge pull request #540 from plajjan/538-user-impersonation-env

Add user impersonation in CLI via env