Fix the notifications icon in alpha

* Fix the bell.svg file to be the correct size
* Move span out of the iconed element in alpha only - restructure
SkinMinervaAlpha to allow this.
* Use a generic class name for notification count.

Bug: 72979
Bug: 72977
Change-Id: I7b4cf7ace757f89eb0b339d9e58eb896e514955e
Merge "Fix the notifications icon in alpha"