In an effort to make `ThreadSettingsModal` easier to work with, we're pulling the `general`, `privacy`, and `delete` tab into their own separate components.
This diff does that for `ThreadSettingsGeneralTab`, but in a limited way. We're basically moving over the `mainContent` JSX markup and prop drilling the state that's still held by `ThreadSettingsModal`.
Once we've extracted the JSX markup for `general`, `privacy`, and `delete`.. and prop drilled as necessary to keep things working as they did before: we're going to push down the state and functionality into the smaller "tab" components so things are properly encapsulated.
And once that's done, we should easily be able to turn `ThreadSettingsModal` into a functional component.. and refactor `ThreadSettingsModal` using @def-au1t's `TabsContainer` and `TabsItem` components.