Synchronize creation of Events and Authors

[mplorentz]

This fixes #446 as well as some problems @martindsq was having with the stories/read branch. It's a different manifestation of the issue described in #385 where we were creating two identical Event records in different threads. In the past we fixed these by ensuring that the two duplicate events were merged into one when the context was saved. This results in one Event on disk, but the two Events in memory would still have different NSManagedObjectIDs until reloaded from the store. This breaks SwiftUI's observation code, meaning that if you were viewing a note or profile for the first time SwiftUI might be observing an objectID that never gets updated even after we download the relevant data from the network.

I fixed it by moving all Event and Author creation onto a single context. It was hard to do this elegantly without needing to pass two contexts (one for creation and one for processing, editing, etc.) all around the app. In the end I used @dependency injection to get the creation context into the creation functions, which meant I needed to change them from class to instance functions. It's a little janky, but I think this is much preferable to the confusion and possible developer errors it would cause to be passing two contexts around.

[mplorentz]

I found a Core Data threading issue while running the unit tests with the Nos Diagnostics scheme. I'll get it fixed and update the PR.

[mplorentz]

I fixed the threading issues but now the unit tests, specifically the SocialGrapheCache tests are taking forever to execute 🤔

[mplorentz]

It turns out using an in-memory persistent store was causing the SocialGraphTests to take a long time. I couldn't figure out why so I switched to using an on-disk store for those. This is now ready for review.