Release 7.4.2: Show folder tree and turn off thread-view by default

[Guest]

Existing OX6 users might be irritated by a hidden folder tree and don't understand the concept of a thread-view. Users might think that all folders are lost and that particular emails are missing.

Affected settings:
io.ox/mail//threadView=false
<app>//folderview/visible/large=false
<app>//folderview/visible/medium=false