Allow removal of language

[mikedingjan]

Issue summary

At the moment when in synced mode, removal of pages isn't allowed, therefore the language can't be removed either, since it's protected by a models.PROTECT foreignkey. The user should get a clear warning that all his pages are going to be removed as well, this should also be handled before removing the language (perhaps by a pre_delete signal).

How to reproduce?

Go to settings > languages and remove a language, at the moment this gives a foreignkey constraint error.

[mevinbabuc]

Also when removing remove the the corresponding roles created too.

[Henk-JanVanHasselaar]

started in pr #100