Make messages easier to read by reducing line spacing in message to separate them from other messages.

### Description

I've been thinking a little this morning about why I find messages in the Riot Desktop app harder to read than other apps, and I think it's mostly that the spacing between lines is the same as the spacing between usernames and content, so it's difficult for me to tell which parts go together, and who's saying what.

I made some tweaks, and changed it from:

to:

which I _think_ we can agree makes it clearer what text belongs with which user (particularly on the top two lines and bottom two lines).

Logs being sent: yes

### Version information

- **Platform**: desktop
- **OS**: macOS
- **Version**: Riot Nightly (2020051401)