Buttons everywhere

- **Section headers** are buttons. `RET` to hide/display, `w` to copy share link.
- **Links** are buttons. `RET` to visit, `w` to copy URL.
- **Code blocks** are buttons. `RET` to open in a separate buffer, `w` to copy the content.

There's also a new _“Add Comment”_ button.

I have plans to make the code-block buffer adopt the right major-mode, possibly even fontify code-blocks appropriately right in the question (something even the website doesn't support right now). But that's for later.