Only configure the latest xwiki syntax by default

Rationale:

• Incitate users to write docs using the latest syntax

Note that existing docs in xwiki/2.0 will work ofc and in edit mode the 2.0 syntax will be displayed.

In addition, always display the syntax select box even when there's only 1 syntax configured. Rationale:

• We now have only 1 syntax by default (xwiki/2.1)
• Makes the feature auto-discoverable
• We need the user to know what syntax the current doc is written in

In order to not make it strange, we also display a link to configure more syntaxes when the user is admin and only 1 syntax is condfigured.