Use system font instead of hard code Roboto

Description

Font is too bold, allow for non-bold font.

To Reproduce

Open chat and use it and observe it is hard to read. Light theme is a little better but dark theme in particular.

Additional information:

• Device: macbook pro
• OS and OS version: Ubuntu 20.4
• Installed version of FluffyChat: Newest as of Nov 13th v0.34.0+git771.7d300ece-dirty