Migrate svn repository to SBForge

This means:

• The Gforge repository will be migrated to the SBForge repository.
• The Gforge repository will be locked for further commits.
• Browsing and checkout of the NetarchiveSuite source will no longer require login.
• The Subversion server will be updated from the current version 1.4 to 1.6. See Subversion 1.5 and 1.6 release notes for improvements.
• The Subversion login credentials will switch to the SBForge credentials. This also means the author commit names may change, if the GForge and SBForge logins differ.

We will switch after the 3.16 release is finished.

See Relocating to a SBForge repository for documentation on the clientside procedure for switching repositories.