Fetch posts on login, purge on logout

[AngeloStavrow]

Closes #13.

⚠️ This PR is dependent on work done in #22. ⚠️

This PR adds an API call (and result handler) to the WriteFreelyModel to fetch user posts from the server, and calls it on login. The handler matches posts that are in a collection to the fetched collections (hence its dependency) and then adds them to the PostStore, which updates the PostListView.

A purge() method is also added to the PostStore, to purge the fetch posts when the user logs out.

[thebaer]

I probably should've mentioned this in #22, but there's actually an experimental, undocumented API feature (originally deployed for the v2.0 Android app) that saves on API calls upon login. When you call the /api/auth/login endpoint, you can supply an additional parameter: ?verbose=1 to retrieve all user collections and posts along with the user data (valid parameters here, and backend logic here).

We should of course support reloading collections + posts separately (once logged in), but during login, I think we should include the verbose=1 parameter in order to save on network calls (particularly on mobile).

Given that information, does this PR need to be changed, or can we still use it as-is for features like #32?

[AngeloStavrow]

That's a good parameter to have!

Because any calls to the WriteFreely/Write.as API are being handled by the Swift package, rather than this client app, I think it makes more sense to implement this as is for now, and open an issue in the package repo to add something like a loginAndFetchContent() method in WFClient. When that's implemented, we can use it here. What do you think?

Once that's implemented in the package, we can add the behaviour to the client. You mention that it's "experimental", @thebaer — is there anything that might need to be guarded against in production?

[thebaer]

That sounds like a good plan!

As for being "experimental," I mostly mention that for the audience, to note we can change this particular interface at any time without notice. But otherwise it's ready and safe to use in production -- we just need to pick which parameter/value we want to make "official" (again, my suggestion: verbose=1).

[thebaer]

Looks good!