- bookmarks: unique title, unique URL, force lang definition
- news (direct to moderation): force section definition, force lang definition on links,
  different messages for max length of news title and link title
- news (in redaction): force lang definition on links,
  different messages for max length of news title and link title
- trackers: for category definition

As title and link are both mandatory, we set the lang mandatory too.

The "xx" language code is removed from the valid list and blank is
refused.

The "xx" value was used as default value before we enable "blank" in
form. It is not really useful now except for links and bookmarks which
are already using this value.

This way, the errors is easier to see at they appear first on screen.

This shows a user friendly message instead of an exception if user
remove the client side validation of section.

The exception were thrown by the preview, so we avoid to show preview
and submit button for invalid news.

The bookmark attribute for URL is "link" and not "url".

The inclusion validator already does the job correctly.