This moves the header, navigation, sidebar, and article toolbar to be
before the content in the DOM. As a result, a lot of absolute
positioning logic can be removed and styles can be simplified.
Note that although the sidebar was moved from the header into the
workspace container allowing it to de-absolutely positioned, its
absolute positioning was kept intact as it has a fair amount of
complexity that should be handled in a separate task.
To activate, set `$wgVectorIsSearchInHeader = true;`
Changes that could cause concern:
* The "jump to search" link was removed as the search is now much
earlier in the DOM and I questioned the value of keeping this. However,
it can be added back in if this change is contentious.
* A "jump to content" link was added to account for the new DOM order.
* Because the sidebar was taken out of the header, users will not be
able to tab from the sidebar button into the sidebar without additional
tweaking (e.g. should we add JS to enable this?). It was deemed that
this work can be saved as a follow-up task.
* I applied `overflow-y: auto` to the `mw-page-container` because the
header's top margin was collapsing and caused whitespace to appear
between the viewport and the header. Alternatively, we could apply a top
padding to the page container and remove the header's top margin. I went
for the simplest solution but am open to alternatives.
* I left the footer as-is in this patch to minimize risk. It might be
cleaner later on to move the footer inside the workspace container which
would leave only one workspace container.
Bug: T261802
Change-Id: Ic553fab3bde25769b103d899b92b3b694c00c384
