Restructure semantic HTML #3252
Description
Currently the HeaderComponent contains the search bar. However, I think it would be better semantics to move the search bar into the <main> https://github.com/projectblacklight/blacklight/blob/main/app/views/layouts/blacklight/base.html.erb#L38-L40. This means that the search form, available filters and results are all enclosed in <main> and not split across multiple elements.
Then we could have a structure like this:
<body>
<header>
</header>
<main>
<search>
<form><!-- search bar goes here --></form>
<section><!-- selected options --></section>
<section><!--facets--></section>
<section><!--search results--></section>
</search>
</main>
</body>