Load urlconfs / aliases from database

[yakky]

To allow fully dynamic configuration we should move MULTISITE_CMS_URLS and MULTISITE_CMS_ALIASES to the database.

• MULTISITE_CMS_ALIASES is trivial as django-multisite Alias already does the job
• MULTISITE_CMS_URLS less so, as we need to add this information somewhere in the models and adding a model just for this, it looks very inefficient

All these information must be cached otherwise we will incur in a huge performance hit