Support for multi speaker collections

[atliSig]

We should create a new db Model for multi speaker collections that would have a one-to-many relationship with normal collections. This would then allow us to treat multi speaker collections differently:

• Multi speaker collections could appear in a special multi speaker dataset list view rather than getting mixed in with all other collections.
• Downloading a multi-speaker collection archive would be simpler since all the data could be generated and downloaded in a single request