Have pages be technical pages by default

We need to make sure that we don't see pages as not hidden when they should be hidden. Thus consider by default that all pages are hidden (ie technical pages).

Also introduce 2 new config parameters:

• visibleTechnicalPages: for technical pages that must be visible but not appear in searches (for ex)
• translatablePages: for pages that can be translated (and thus have their default language that must not be empty)

See also https://github.com/xwiki/xwiki-platform/commit/cc86017d0a59e37756c78057254623292a155655#commitcomment-38575742