Make it possible to optionally hide the "View Legacy XImgView" link in all sessions in an XNAT

[Will Horton]

The Image Viewer code includes Velocity templates that overwrite the old XImgViewer link in the Actions box, and adds the link to the new Image Viewer. It’s a bit confusing to have both links, but I can see where some systems might want both (I guess?). It should be fairly easy to create an admin setting that allows a site admin to hide the old link all together. An approach:

1. Create a new siteConfig setting called “hideLegacyViewer” and set it to “false” by default
2. Create an admin panel where a Site Admin can toggle this setting
3. Add a wrapper in Velocity around the legacy link to check the value of this config setting and hide the link if needed