Draft: Superevent Log Inheritance from Events

The goal of this MR is to implement superevent Log Inheritance. Basically, it's a way to get around downloading files from events, just to re-upload them to superevents. I'd like to have the inherited logs have the same properties ('filename', 'comment', 'fullpath') as the source log, and also inherit the original tags (like 'sky_loc'), and also get new tags from the superevent (like 'public') and have the templating system "just work" the same way.

• Create model (foreign key to superevent and source log object, manytomany for superevent tags, combined queryset for tags). Note to self: should filename, datadir be added to the db table? It would make it queryable.
• Expose API endpoint for adding/removing
• Figure out file handling and linking. I think it would be the least painful to create a symlink in the superevent folder that points to the version in the event directory. This avoids the need to expose g-event links on public tags, and lets the superevent permissions do the work.
• Incorporate into page views and log lists
• Implement inheritance logic (using a log's N number? retrieve by tag(s)?)
• Implement in client code
• Add stuff to pipeline preferred table.