This will automatically enable the glimmer header when all installed themes/plugins are ready. This replaces the old group-based site setting.
In 'auto' mode, we check for calls to deprecated APIs (e.g. decorateWidget) which affect the old header. If any are present, we stick to the old header implementation and print a message to the console alongside the normal deprecation messages.
To override this automatic behavior, a new `glimmer_header_mode` site setting can be set to 'disabled' or 'enabled'.
This change also means that our test suite is running with the glimmer header. This unveiled a couple of small issues (e.g. some incorrect `aria-*` and `alt` text) which are now fixed. A number of selectors had to be updated to ensure the tests were clicking the actual `<button>` elements rather than the surrounding `<li>` elements.
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