Move all the LESS `z-index` settings to variables.less so it's easy to
conceptualize how UI will be layered. jQuery CSS rules are untouched but
documented.
Also, remove the seemingly redundant re-setting of the `mw-searchButton`
`z-index`:
#mw-searchButton
z-index: 1;
}
This rule was added in 0c77e4f.
The sidebar work will require `z-index` properties and this refactor
eases the comprehension of those changes, as well as prevents the
accrual of more `z-index` technical debt.
Bug: T246419
Change-Id: Ic112a0ee3f701f87432838797be45c6069fdb522
b80ad85bb1