Remove WAGTAILTRANS_LANGUAGES_PER_SITE setting

[mikedingjan]

In the early days of wagtailtrans we've created a setting which allowed users to define the default and other languages of a wagtailcore.Site this behavior is very common when dealing with multiple sites.

To have a better maintainable codebase it would be great to have this as default, since it can also be used when having an instance with a single site. I think next to maintainability we will also be able to introduce some performance benefits with this.

Going forward with this we need to proper document this and have a data migration which handles this for non site language sites.

Steps to take:

• Remove WAGTAILTRANS_LANGUAGES_PER_SITE setting
• Remove Language.is_default, this is defined via sitelanguages
• Create data migration
• Update unit tests
• Update documentation