Changing text/char fields in EMBBEventLog model to blank=True as appropriate, also adding N/A option for observation status field.

Made changes to store permission info on the Event objects themselves. This speeds up some searches by an order of magnitude with respect to guardian's get_objects_for_user.

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.