Mention which 'role' is missing when operation returns 'not allowed'
Reuse Operation.requireRole() for add/edit news operations
Add tests for add/edit news operations
Drop unused import in operation/news.py
Delete unused functions and imports from page/news.py
Fix height of "Add news item" textarea so it looks OK in chromium

This makes the textarea a few pixels lower which is enough to
prevent a scrollbar from appearing for the dialog client area.
A similar fix was done for "Create Comment" dialog in af7cb57.
Quirk to avoid vertical scrollbars for resized dialogs in chromium

Without this quirk the height of the textarea gets too large when
the dialog is resized, making a vertical scrollbar appear for the
client area in the dialog. Quirk is similar to 370912f.
Reload news page after adding a news item

This makes the new news item immediately visible and shown as
unread so it's more obvious that posting the news item worked.
Give primary admin newswriter privileges by default