Standard line height and maring in all content HTML texts.

This fixes
https://otrs.hallowelt.biz/otrs/index.pl?Action=AgentTicketZoom;TicketID=6532#62734,
where font size and margins would be different in content tags like DIV
and P.

Change-Id: I6abcbeb3d531effca89026f2d4c557868c4be06e