ENH: Add ability to install extension from file

Hook up 'install from file' action in extension manager. Create an
overload of qSlicerExtensionsManagerModel::installExtension taking only
an archive file name, which uses the new list_archive method to
determine the extension name and then hands off to the existing method.
Modify the existing overload to cope with the lack of server metadata
that occurs in this case, and to fill in a sufficient subset from common
information (i.e. the os/arch/revision of currently running Slicer) and
the description file provided in the extension archive.

This should allow users to install extensions from externally obtained
archives of the same (e.g. manually downloading from the server with an
external web browser).

Issue #2145