Implement Subscriptions endpoint

[Sporiff]

A subscription defines the link between a user and a podcast. The subscription should contain minimal information about this interaction such as:

• A timestamp representing the datetime at which the link was made
• A timestamp representing the datetime at which the link was last updated

podcast metadata MAY be stored alongside the subscription or separately, as long as the correct response format is sent. The guid value for the subscription MUST match the guid of the podcast. This value MUST be calculated by the client and sent to the server. The server MUST NOT attempt to validate this value beyond checking it is a valid UUID as the feed_url used for calculation may change after the guid is assigned.

The subscriptions endpoint needs to support the following features:

• GET and POST actions for bulk fetch
• GET and POST actions for individual resources

All subscriptions should be scoped to the requesting user.