Manga cover updates (#3101)

* cover caching overhaul

* add ui for removing custom cover

* skip some loading work

* minor cleanup

* allow refresh library metadata to refresh local manga

* rename metadata_date to cover_last_modified

* rearrange removeMangaFromLibrary

* change custom cover directory
add setting for updating cover when refreshing library

* remove toggle and explicit action for updating covers

app/src/main/java/eu/kanade/tachiyomi/ui/browse/source/browse/BrowseSourcePresenter.kt

@@ -28,6 +28,7 @@ import eu.kanade.tachiyomi.ui.browse.source.filter.TextItem
 import eu.kanade.tachiyomi.ui.browse.source.filter.TextSectionItem
 import eu.kanade.tachiyomi.ui.browse.source.filter.TriStateItem
 import eu.kanade.tachiyomi.ui.browse.source.filter.TriStateSectionItem
+import eu.kanade.tachiyomi.util.removeCovers
 import rx.Observable
 import rx.Subscription
 import rx.android.schedulers.AndroidSchedulers
@@ -279,7 +280,7 @@ open class BrowseSourcePresenter(
     fun changeMangaFavorite(manga: Manga) {
         manga.favorite = !manga.favorite
         if (!manga.favorite) {
-            coverCache.deleteFromCache(manga.thumbnail_url)
+            manga.removeCovers(coverCache)
         }
         db.insertManga(manga).executeAsBlocking()
     }