feat: make thread elements have the same space around them

[GretaD]

fixes #11316

Only worked on these points as added by Jan on the ticket:

• The mail heading & participants should be aligned to the content box.
• and the "Reply" button as well as the action buttons on the top right
• When scrolling the content, the container / top divider line vanishes and gets overlapped partly by the heading, making it look strange
• Now with the message list having more content, horizontal divider lines could make sense to have some nicer separation.
• Mails have different heights in the message list if there’s shorter ones – but all should have the same consistent height
• Whitespace is sometimes not the same across the board, for example the space left and right in the message list is much less (half?) than in the left navigation.

before

after

after(reply button aligned with the action menu)

Subject and recipient aligned with the recipient in the message header. plus added the line to split subject from the message

more space for the preview, but this is a bit tricky to add more space for the preview because the details, the unread and the action menu wouldnt allow it to go further.
before

after