Moving some of the chat-thread-list.css css to chat-thread-list-item-menu.css as a way to separate some of the CSS we're using in multiple places.
The goal is to isolate the CSS so we don't break functionality in other parts of the app.
At some point down the road, we should break up all of the css files and remove replicated CSS. https://linear.app/comm/issue/ENG-868/break-out-thread-list-css-into-their-own-files