Creates two new templates:
- ContentHeader.mustache containing the site header and indicators or languages
- ContentSubheader.mustache containing site tagline and conditionally indicators
This changes DOM order.
- It adds the `<header class="mw-body-header">` to all pages,
including special pages & edit pages, not just pages with the language button.
- It moves the #siteSub element (tagline) outside of the
`<div id="bodyContent">` element.
- Adds a clearfix (or margin-collapse fix) to `.vector-body`.
This changes causes a minor difference (perhaps improvement) where on special pages,
the beginning of the content, starting with the `mw-specialpage-summary` element,
was positioned closer to the page title than on regular pages.
The clearfix ensures that the height between the page title and content is consistent across all pages.
Bug: T303549
Change-Id: I29ba337dbd6f9b0cf58c9a0ca517a9ba59cef414
