Move VectorBeta modules to VectorBeta extension

* Remove duplication of styles in skins.monobook and skins.vector by
  creating skins.common.
* At same time allow VectorBeta to replace Vector-specific skin styles
  without having to worry about common styles.

Dependency: Ie32a28f8f8c5fc8e4d0890a3d7d96bc82b39c481
Change-Id: Ia193571afffff36ecb1f14ee9036fb9bb98347e7
Merge "Move VectorBeta modules to VectorBeta extension"