Add support for using the user's chosen Kitsu rating scales (#3818)

Add support for different Kitsu rating scales

Includes a database migration because we have been scaling Kitsu's
2-20 `ratingTwenty` integer value to a 1-10 (step 0.5) value and
stored that in the database.

I'm also "rounding" the values to the next lowest valid one in the
current rating system to mirror Kitsu's behaviour for this. Both Kitsu
and the app keep the misfit value around until the score is otherwise
edited, at which point the interface constrains the user selection to
only valid values (for the current system).

Maybe the RatingSystem data class is overkill, but a previous version
of mine had three constants for each system that had to be manually
associated at each corner. Encapsulating those in a little map of data
classes seemed the more ergonomic solution.

CHANGELOG.md

@@ -13,6 +13,7 @@ The format is a modified version of [Keep a Changelog](https://keepachangelog.co
 ## [Unreleased]
 ### Added
 - Add `id:` prefix search to remaining trackers (AniList, Bangumi, Kitsu, MangaUpdates, Shikimori, and Hikka) ([@MajorTanya](https://github.com/MajorTanya)) ([#3776](https://github.com/mihonapp/mihon/pull/3776))
+- Add support for using the user's chosen rating system for Kitsu ([@MajorTanya](https://github.com/MajorTanya)) ([#3818](https://github.com/mihonapp/mihon/pull/3818))
 
 ### Improved
 - Show updates and upcoming filter icon as active for categories ([@Secozzi](https://github.com/Secozzi)) ([#3772](https://github.com/mihonapp/mihon/pull/3772))