* Collate browser header concerns into component
Header component has a slots field with a left, center, right, and a
right-most location for a current account, which will link to an account
switcher.
* Use feature flag if OK for production eventually
* Make sure centered content centered
* Allow for disabling header theming for login page
visual gitches exist for links and buttons, due to specifications futher down in the header, but those
items shouldn't use the `no-theme` option. For now, it's just for the
login screen
* Select sr-only headers
* Fix vault-filter center header component
* Correct hover for header buttons/links
* Ensure no visual difference when flag is off
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