Improved thumbnail handling

- ThumbnailsModel optionally returns the quality
- Have constants for 4:3 and 16:9 aspect ratio
- Add high quality options for thumbnails
- Rename Highest quality to Best quality
- make 4:3 thumbnails fill the VideoCell
- use .maxes instead of .maxresdefault (the latter sometimes returns white images)

Signed-off-by: Toni Förster <toni.foerster@gmail.com>

Shared/Defaults.swift

@@ -462,12 +462,14 @@ enum ButtonLabelStyle: String, CaseIterable, Defaults.Serializable {
 }
 
 enum ThumbnailsQuality: String, CaseIterable, Defaults.Serializable {
-    case highest, medium, low
+    case highest, high, medium, low
 
     var description: String {
         switch self {
         case .highest:
-            return "Highest quality".localized()
+            return "Best quality".localized()
+        case .high:
+            return "High quality".localized()
         case .medium:
             return "Medium quality".localized()
         case .low: