Optimise seen post/reply detection

[bakape]

• [ ] Consider everything in the same thread with a post ID lower than the lowest scrolled to post as seen. This would remove the need for massive DB write spam enteriley at the cost of less granularity.
• [ ] Ignore inlined posts
• [ ] Disable on boards
• [ ] Disable in gallery mode
• [ ] Drop DB storage needed for previous functionality
• [ ] Patch inter-tab broadcast
• [ ] When scrolling to bottom of last thread page, mark the last post in the thread as read. Do this to account for inlined posts possibly being higher up in the DOM tree.