[ticket/11111] Prosilver posting textarea may only be resized vertically

The resize element displays a mechanism for allowing the
user to resize the element. Setting this to vertical means it
may only be resized vertically.

PHPBB3-11111
Merge remote-tracking branch 'vsephpbb/ticket/11111' into develop

* vsephpbb/ticket/11111:
  [ticket/11111] Prosilver posting textarea may only be resized vertically