Better styling for updated Twitch chat.

I updated the UI to make use of the new CSS styles such as `chat-settings` and `chat-menu`. It now looks like this:
![](https://hostr.co/file/CUBlAt64f44v/TwitchChatFilter-UpdatedUI.png)

I also made it easier to add the different sections (hide, rewrite and style) through `add_section`, which then calls `add_option` on every option in the section.

I changed some `.after` and `.append` calls into `.insertAfter` and `.appendTo` so we can more easily work with the inserted elements as variables (instead of having to rely on IDs and querying them again).