Post search filter UI offers sort types that are unavailable in 1.0

[Sjmarf]

Pre-1.0, the available sort types for post searching are represented using the PostSortType enum. From 1.0 onwards, the SearchSortType enum is used instead. Currently, Mlem always offers the PostSortType cases, regardless of which Lemmy version you're using. This needs to be changed so that it only shows the sort types offered by Lemmy 1.0 (aka the SearchSortType cases) if the user is logged into a Lemmy 1.0 instance.