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.

Depends on D5553

Restart both keyserver and web with yarn dev, and there are no errors in the logs. Also manually checked each of the imports to make sure that they were all correct and up-to-date. Lastly, I
ran flow in the console within web to ensure that there were no flow-related issues that came out of this change.