Feature/decouple thumbnail downloads and cache (#581)

* Rename "DownloadedFilesProvider" to "ChaptersFilesProvider"

* Move files into sub packages

* Further abstract "DownloadedFilesProvider"

* Rename "getCachedImageResponse" to "getImageResponse"

* Extract getting cached image response into new function

* Decouple thumbnail cache and download

* Download and delete permanent thumbnails

When adding/removing manga from/to the library make sure the permanent thumbnail files will get handled properly

* Move thumbnail cache to actual temp folder

* Rename "mangaDownloadsRoot" to "downloadRoot"

* Move manga downloads into "mangas" subfolder

* Clear downloaded thumbnail

server/src/main/kotlin/suwayomi/tachidesk/manga/impl/ThumbnailDownloadHelper.kt

@@ -0,0 +1,23 @@
+package suwayomi.tachidesk.manga.impl
+
+import suwayomi.tachidesk.manga.impl.download.fileProvider.impl.ThumbnailFileProvider
+import java.io.InputStream
+
+object ThumbnailDownloadHelper {
+    fun getImage(mangaId: Int): Pair<InputStream, String> {
+        return provider(mangaId).getImage().execute()
+    }
+
+    fun delete(mangaId: Int): Boolean {
+        return provider(mangaId).delete()
+    }
+
+    suspend fun download(mangaId: Int): Boolean {
+        return provider(mangaId).download().execute()
+    }
+
+    // return the appropriate provider based on how the download was saved. For the logic is simple but will evolve when new types of downloads are available
+    private fun provider(mangaId: Int): ThumbnailFileProvider {
+        return ThumbnailFileProvider(mangaId)
+    }
+}