Added migrations for gdb_pe robot cert and 'background' tag. Also added 'background' to the list of blessed tags in the settings module.

Fix for Issue #975, 'Versioned files displaying only newest versions in event detail page'. To fix this, we add a file_version field to the event log. This is accessed from the VersionedFile object at the time of file-saving. That way, the correct version can always be associated with any given log message.