Feature/download ahead trigger chapter downloads client side (#597)

* Trigger download ahead while reading client side

* Trigger download ahead while reading earlier

Download ahead was only triggered once the chapter got marked as read

* Optionally ignore dupe chapters for download ahead

* Consider current chapters scanlator for dupe check

* Update download ahead related settings

- add new metadata setting for download ahead limit
- previous setting
  - rename
  - update usage (auto download new chapters limit)

* Select chapter to auto delete while reading depending on ignore dupe setting

Currently, the n-th to last chapter always got deleted ignoring if "ignore dupes" setting was enabled or not.

* Only auto delete chapter while reading if it is read

In case not the last read but the n-th to last read chapter should get auto deleted while reading, it currently just got deleted ignoring if it has been read or not.

src/util/metadataServerSettings.ts

@@ -15,6 +15,7 @@ export const getDefaultSettings = (): MetadataServerSettings => ({
     deleteChaptersManuallyMarkedRead: false,
     deleteChaptersWhileReading: 0,
     deleteChaptersWithBookmark: false,
+    downloadAheadLimit: 0,
 
     // library
     showAddToLibraryCategorySelectDialog: true,