Notes should only store ids and urls of other services

[viamin]

For example, for a task synced between Omnifocus and Asana, the Omnifocus item should have the Asana ID/URL in the notes, but the Asana item should have the Omnifocus ID/URL in the notes.

Currently, I'm seeing only the Asana ID/URL on both items (the Omnifocus info is never added to notes)

[viamin]

This might be easier if a new object is added - something like SyncPair or SyncCollection, which contains all of the known sync items from all of the different services. Currently it's stored in a Set called item_pairs but it doesn't have much functionality. This could also potentially prevent any chained sync issues (which could also fix #128)

[viamin]

I wonder if also adding a sqlite database will make tracking this easier - I was trying to avoid that in the past, but this route would make a lot more sense since a collection wouldn't need to be built up each sync. I think this could also reduce some calls, since we can start storing timestamps per item, rather than for each service.