Check 'useSystemTheme' in quick settings theme switcher (#7809)

* mock Element.scrollIntoView in jest setup

Signed-off-by: Kerry Archibald <kerrya@element.io>

* extract theme switcher from quick settings, add match system option, test

Signed-off-by: Kerry Archibald <kerrya@element.io>

* i18n

Signed-off-by: Kerry Archibald <kerrya@element.io>

* forgotten copyright

Signed-off-by: Kerry Archibald <kerrya@element.io>

* stylelint

Signed-off-by: Kerry Archibald <kerrya@element.io>

* remove old class

Signed-off-by: Kerry Archibald <kerrya@element.io>

src/i18n/strings/en_EN.json

@@ -1112,9 +1112,9 @@
     "All settings": "All settings",
     "Pin to sidebar": "Pin to sidebar",
     "More options": "More options",
+    "Settings": "Settings",
     "Theme": "Theme",
     "Space selection": "Space selection",
-    "Settings": "Settings",
     "Delete avatar": "Delete avatar",
     "Delete": "Delete",
     "Upload avatar": "Upload avatar",