Add MLEB (MediaWiki language extension bundle) role

This patch add a new role for the MediaWiki extensions below:

* Universal Language Selector.
* Babel
* Localisation Update
* Clean Changes
* Translate
* Interwiki

The CLDR extension was moved to a separate class, 'role::cldr', so that it may
be required by both MLEB and education roles.

Change-Id: Idac0215c17cd67ade888a85494fc8bf7af21d4f0