There is currently a "require" chain between `keyserver` and `web/markdown`, where it makes it impossible to use css files within the directory since `keyserver` is unable to handle it. Context:
https://linear.app/comm/issue/ENG-2206/extract-useonclickpendingsidebar-from-webselectorsnav-selectors#comment-31e14467. This means that in order to get around this, we need to break this "require" chain to
enable the use of `css` files in `MarkdownSpoiler`. This diff handles that - we separate the thread-like selectors out of `web/selectors/nav-selectors` into a new file, `web/selectors/thread-selectors`, and
fix the import statements around the codebase. This allows this chain to be broken and, after testing, `MarkdownSpoiler` importing a css file throws no error.