[Backport to 12.0] Bottom toolbar of HTML fields is shown by default

The bottom toolbar of HTML fields in the Visual Editor is shown by default. But that toolbar takes away unnecessary space so it should be hidden by default. Only when plugins populate the bottom toolbar is should be explicitly enabled.

This behavior has been changed in master (12.1) already in CHANNELMGR-1445, which has been backported already to 12.0 in CHANNELMGR-1446. However, that backport did not include the change in the CKEditor code, which contains the default configuration value. That change should be backported too.