Fixed in r21897
See http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=21897

// ----------------------
BUG: External link in extension manager now open in browser. Fixes 0003045

To determine if a link should be open in the extension manager or in an
external browser, the "host" of the clicked url and the server url are
compared.

See http://qt-project.org/doc/qt-4.8/qurl.html#setAuthority for more details.

See http://www.na-mic.org/Bug/view.php?id=3045

Fixed in r22347
See http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=22347

// -------------------------
BUG: Fix opening of link in extension manager

Opening of links different from slicer.kitware.com host into regular
browser has been initially introduced by r21897. Then, when commit r22220
introduced qSlicerWebWidget, the management of external links has been
broken.

This commit fixes the issue by ensuring the qSlicerWebWidget always open
link within its associated webview. The slot "onLinkClicked" from
"qSlicerExtensionsInstallWidget" will check what is the host and open
into a regular browser if needed.

See 0003045

Fix committed to 2145-support-for-installing-extension-from-file branch.