Structural change to prepare for CSS grid usage.
ArticleToolbar component is defined and self contained with
all layout rules relating to toolbar
UI regression testing suite shows 6 failures due to pixel alignment
changes to tabs. When comparing to the current release, you should
see 34 failures before /and/ after this change.
Notable changes:
- #mw-head element is no longer present in modern Vector skin
to allow us to distinguish between cached HTML and modern HTML
Bug: T303484
Change-Id: Ia261ea53931217f32a21b33c2ab3d4ec64efa48a
