Make locale site aware

This commit is a first step in making the locale module site aware

* New site_id field in locale table
* Save site_id when doing CRUD on locale
* Save imported locale under the main sites locale
* Add site_id in Locale cache name
* Fetch the right locale

To do:

* Fall back to the main site locale (to make inheritence possible)
* Test every possible method to CRUD locale
* Add message to show if you're editing main site or other locale
* ...

All this stuff is done in one commit because all these changes were
highly coupled and I didn't want to commit code that didn't work to make
the steps smaller.