Separate message text font from UI font

### Description

I believe it's imperative to allow a different font for the UI, and another font for the text of messages.

This is what's customary in essentially all applications which allow font control - no reason for that to be different for Element. Specifically, it will allow the UI to be harmonious with the desktop environment and the rest of the windows, while we can choose, say, a monospace font for our messages, to be in line with IRC custom and for easier (IMHO) reading.

The one-font-for-everything also means that you can't increase the message font size for easier reading without making most UI elements larger at the same time.

### Version information

- **Platform**: Desktop

For the desktop app:

- **OS**: Devuan GNU/Linux 3 Beowulf
- **Version**: 1.7.1