Imaging browser permissions

Three permissions available for the imaging browser:
- imaging_browser_view_site      = can view only own site's imaging sessions 
- imaging_browser_view_allsites = can view all sites' imaging sessions
- imaging_browser_qc                 = can QC imaging sessions

If user does not have QC permission, he/she can still see the QC feedbacks and statuses.
Thank you for checking out the pull request :).

The changed line is 961 (renamed feedback_mri to imaging_browser_qc) and the added lines are the two last lines. Everything else is just spacing. I can rebase it, but the last two lines won't be aligned with the rest of the INSERT statement (was not using the "tab=4 spaces" convention we have now).

Let me know what you prefer.