[Room list] Update the font of the sender message

[Florian14]

Currently, the font and the text color of the sender in a message preview are too close to the room title.
We have to realign this to the Element design (ie. remove the bold from the sender style).

[areox-net]

For semplification, we can keep the same look as the Element base: the name of the author of the post is the same font-weight as the message, only in a darker color.