DRILL-7203: Accept impersonation userName as form field & fix back button for query page
Requiring the impersonation username to be sent as an HTTP header made
it difficult to implement the Web UI submission. This changes it so
that the userName can be sent as a form field instead, allowing the Web
UI to do a "normal" form submission, which fixes the back button, avoids
the use of the deprecated "document.open" API call, and may avoid other
glitches in the future.
closes #1994
