Tidy up mail UI

[marcoambrosini]

• Overflowing text and alignment problems in mobile view (threads) #9752
• In "list layout", add a breakpoint and switch back to the 3 line element when the screen becomes too narrow #9753
• Use secondary button for new message #9754
• Tidy up search bar #9755
• Mail header has too much top and bottom padding #9786
• Only show border of outbox mailbox when scrollheight > clientheight
• Use NcNavigationCaption for account navigation element. Remove color circle #9756
• Less space around priority inbox
• Outbox has the wrong font colour when active #9707

Other small fixes that came up in internal design feedback calls:

• In the composer, change the icon for toggling the formatting bar to format color text icon #9823
• Move the attachment items to a separate menu outside the 3 dot menu #9804

In the end it would look like this