Bug 29042: Improve formatting of entry form in Additional Contents

Bug 29042: Improve formatting of entry form in Additional Contents

This patch makes some corrections to the Additional Contents template so
that the content entry form can be styled like other similar forms in
the staff interface.

The patch also updates the form's "title" field so that it is longer and
has a maxlength attribute matching the size of the table column.

To test, apply the patch and go to Tools -> News.

 - Create a new news item.
 - In the entry form, confirm that the "Title" and "Content" fields are
   styled consistently with other similar forms.
 - Confirm that the "title" field is longer.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>