Improve MAL library import

- `updated_at` was previously set to `my_last_updated`, which doesn't exist in exported XMLs. It's now set to `Time.now`.
- `notes` now prioritizes `my_comments`, then falls back to `my_tags`.
- Added `reread_count` for manga, `rewatch_count` and `rewatching` for anime. There's a bug where `my_times_watched` can be `255`, so I added a check for that too.
I assumed that `mal_entry[:rewatching]` is a boolean value. Isn't it?