Bug 29265: Add option to pick editor when creating a new content entry

Bug 29265: Add option to pick editor when creating a new content entry

This patch changes the "New entry" button on the additional contents
page to be a split button which offers the alternative of the
non-default editor (as defined by AdditionalContentsEditor).

Also changed: I moved some of the template logic around to consolidate
handling of the editor variables.

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

 - At the top of the page there should be a split button. The dropdown
   option should offer the non-default editor.
 - If AdditionalContentsEditor is set to "text editor," clicking the
   "main" part of the button should take you to an entry page which uses
   the text editor.
   - The dropdown part of the button should say "New entry using
      WYSIWYG editor."
 - Switch the AdditionalContentsEditor preference and confirm that the
   button reflects the change.
 - Confirm that the individual "Edit" buttons for each existing content
   entry still works correctly according to the system preference.

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>