Feature request: Bookmarking comments

This feature allows users to do many things without using external tools outside of the website. They can bookmark comments with content they find useful, save comments they don't want to reply to immediately for later (which helps keep notifications clean instead of leaving them marked as unread), and generally keep anything they're planning to revisit.

A "Bookmark" button can be added alongside the comment actions (above "Comment link" maybe). A new /bookmarks page can also be added, accessible using a "Bookmarks" button under "User content" in the navigation menu.

On the bookmarks page, bookmarked comments can be displayed in the same way as filtered comment results, with a link above each one to visit the thread and stuff. Another little idea is that the bookmark icon can appear next to the mute icon for bookmarked comments.