`manga.release` shouldn't be persisted

Because "Today" or "Yesterday" is a relative date and becomes out-of-date very quickly. Not persisting it depends on https://github.com/agilgur5/mst-persist/pull/16 .

That being said, we could add some logic to just _not render_ it if the manga is a favorited manga for now, which fixes the most confusing part of the bug (having your manga say it was updated "Today" when it was actually released several days ago). `manga.release` was really only meant for latest manga, and I believe it's only persisted when it a favorite overlaps with a latest (as I think the updated date is only available on the latest page).

Sometimes `manga.release` is an actual date, and then it appears in gray, which is actually interesting (though could still be out of date). An interesting feature could be instead to display `manga.chapters[0].date` when there are no new unread chapters; I think that's better/more informative than just showing nothing. And thanks to #24 's auto-fetch chapters logic, it is always up-to-date (except when no internet connection) for favorited mangas. We could also update `manga.release` to just be a cached version of `manga.chapters[0].date` as well.
For latest manga, their chapters likely haven't been fetched yet, so they should still display "Today" / "Yesterday".

In theory, there's a different fix that actually isn't as difficult (but would likely still require a migration), which is just to change `manga.release` to `types.maybe(types.date)` and parse "Today" or "Yesterday" (if it's only those two, that is relatively easy)