Fixes#3494
Removes `class="next-head"` from the children of the `<Head>` component. Instead, a single sentinel meta element named `next-head-count` is appended. The content is the number of contiguous elements immediately preceding the sentinel that _would have had_ the `class="next-head"` attribute.
During an update, instead of searching for `class="next-head"`, the sentinel is located and the N previous elements are considered candidates for `oldTags`. New elements are inserted before the sentinel, and finally the sentinel is updated to reflect the new count.
e68307df3a