Ps/pm 2910/browser header component (#6641)

* 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

apps/browser/config/base.json

@@ -2,6 +2,7 @@
   "dev_flags": {},
   "flags": {
     "showPasswordless": true,
-    "enableCipherKeyEncryption": false
+    "enableCipherKeyEncryption": false,
+    "accountSwitching": false
   }
 }