Avoid using positive tabindexes

Avoid using positive tabindexes

The use of positive tabindexes is widely discouraged. Instead, we
should order the elements of the page in the same order as they
are displayed, which they are already.

Previously the tab indexes did match the order that these elements
were shown on the page, but the lack of tabindexes on certain other
elements was used to "skip over" things like explanatory links
in forms. This made these other elements hard to focus, and meant that
overall the focus skipped around the page unintuitively.

It is less confusing for keyboard users if the focus just moves around
in the same order that the elements are shown on the page, which is
also the same order they appear in the html.

See https://herb-tools.dev/linter/rules/html-no-positive-tab-index
and the list of references on that page, for further discussion.