Fix layout of message list until we move to component

[jancborchardt]

This will eventually be fixed when we use the AppContentListItem component, cc @ma12-co nextcloud-libraries/nextcloud-vue#616 (This is also essentially a mockup of how this should look like in Mail)

Before:

• Avatar small and not vertically centered
• Second row is too low and falls off
• Action icon hover/focus feedback is outside of element

Now:

• Nice vertical alignment
• Basically same layout as list in Contacts
• Menu hover/focus feedback fits into entry
• The only thing a bit off is that timestamp text and icon don’t right-align, but that’s something for another time, or possibly not fixable since we need the clickable area.
  

[skjnldsv]

Looks good!