Share support

[NattyNarwhal]

• Support Subsonic's shares API, manage shares for a thing
• Do we need to include them in Core Data?
• Use system share sheet to make sharing links easy

[NattyNarwhal]

The common pattern for most things currently is that a SubsonicParsingOperation will put things into core data and fire off a notification when done. For things that last like tracks, this is OK, but a share is usually more transient as it can expire. I don't know if that deletes it on the Subsonic end.

The annoying part is the UI flow is different for local (just show share sheet) vs. server (have to create a share link first, with another dialog).