Updating user avatar icon to overhauled standard WikimediaUI icon.
Also:
- change `line-height` of personal menu to `14px` for better positioning
and introducing corresponding LESS variables
- slightly increase user avatar icon to be on par with ULS one
- putting avatar icon within link on active user, an awful user experience
that has long bugged me.
Bug: T123810
Change-Id: Iabde041090a87968b5f82e36fd81419083f80984
