Migrate old iSub 3 data model to allow for seamless upgrades

[einsteinx2]

iSub 4 will be a direct upgrade to iSub 3 but uses a totally different data model.

Items to migrate:

1. Servers must migrate so the app can continue to connect
2. Cached songs must be available as some users have very large caches.
   a) Due to differences in the data models, they may need to be placed into a separate area in the Downloads tab and may not be possible to connect them with song entries in the new database. However, if there's any way to make this work, we should associate them properly
   b) All currently downloaded songs should not be considered pinned
   c) Images will need to be re-cached
3. Local playlists should be converted to new DB format and synced to server
4. Bookmarks should migrate (may have similar issues as cached songs, but hopefully we can make it work)
5. We should be able to throw out everything else