Don't rewrite the urls for assets loaded via `url()` in CSS.

Instead, they should be relative to the CSS file.

Also, listen to the `focusin` and `focusout` events, since they bubble,
allowing us to listen on the chatbox itself.

before creating any new ones

Only load it once the first chat has been opened.

This will make it easier to add config-based functionality, such as
allowing/showing the `/topic` slash command only to those users who are
allowed to set the subject.

The .chat-content are doesn't resize when the textarea gets resized to
its original size once a message gets sent.

- Use flexbox where possible
- Simplify header by using `justify-content` instead of setting column widths
- Show description below the row containing the title and buttons