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.
My list has some entries where this value is erroneously set to 255. Heard about similar issues from other people before, so I added in a check for it. I went ahead and used `<` instead of `!=` but I don't think it's going to make any difference.