XNAT-2834: Knocked level of unknown field log statements down to info to prevent flooding log files.

Committed session builder changes from this changeset: https://bitbucket.org/nrg/sessionbuilders/commits/aba6a60f0e0f25c7c550182455f4131820ff9178
XNAT-2888: Added check for empty file writers list. If no writers are found in PUT/POST, the available size of the request entity is checked. If that's 0, user's informed something's not right. If it's non-0, user is informed of possible work-arounds.
XNAT-2897: Added ability for admins to get session count for other users.