Improve email side panel design

[Bonapara]

Current behavior

Text is hard to read in the side panel (too compact)

We can see the email thread under the message:

Desired Behavior

Line height should be 140%

Make sure it is the case for all Base and Base/small fonts.

Remove the email thread under the message

In Front (Front app), a toggle is displayed. If possible, we should display nothing for V1.

Mail content Margin bottom

Add a 8px margin-bottom:

[FelixMalfait]

@Bonapara fyi in this issue some issues are related to parsing (backend) and would be quite difficult and time-consuming (also need to confirm is the strategy is always to display raw text and not html?), while others are small frontend fixes. Maybe it's worth separating this in two?