Persist content locally

[AngeloStavrow]

Closes #16.

This is A Big One™ —local storage for your user collections and posts— which gets us much closer to a usable client, and it touches nearly every file in the app (plus adds a few new ones).

I'm a model, you know what I mean

We start by replacing the Post and PostCollection custom types with WFAPost and WFACollection managed objects — CoreData model objects for posts and collections, respectively. These are created in LocalStorage.xcdatamodeld and the extension classes are generated manually (Editor > Create NSManagedObject Subclass...).

Because NSManagedObjects conform to ObservableObject, we can use these directly in our SwiftUI views, and rather than maintaining an in-memory store (e.g., PostStore's posts array), use FetchedResults to populate (and update) our lists of collections and posts.

I insist we persist

We continue by adding a shared, top-level LocalStorageManager. This instantiates and initializes our persistent container as a (static) class property, adds an observer for notifications sent when the app resigns its active state, and adds a helper method for saving changes to the managed object context.

1-800-RIP-IT-OUT

We finally go through the codebase, replacing anything related to the Post and PostCollection types with WFAPost and WFACollections. There are two main concerns here:

1. Maintaining state through the views: Rather than passing in State / Binding objects, we can —when necessary— tap into the managedObjectContext Environment property wrapper in our views, which lets us create, for example, FetchRequests for populating lists from FetchedResults. As a result, we don't need a PostStore type anymore, so this gets converted into a PostListModel type.
2. Translation in the WriteFreelyModel: Previously, the Post type contained a WFPost property — a custom type from the WriteFreely Swift package, which handles requests to the WriteFreely API; without this, the WriteFreelyModel public API must create a WFPost object from the (local) WFAPost's property to hand over to the Swift package.