Bug 29828: Prevent additional content to be created without default

Bug 29828: Prevent additional content to be created without default

When creating or editing an additional content, the form is submitted if
only the title of the 'default' tab is filled in.
We should submit the form only if both title and content exist.

Test plan:
Create a new content using text editor (codemirror)
Confirm that the form is submitted only if title and content from the
default tab are not empty
Try again with wysiwyg (tinymce)

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>