Cleanup tasks in the Settings code (#12125)

* inline call to `SettingsStore.isEnabled`

* Remove confusing `SettingsStore.isEnabled`

This was basically the same as `SettingsStore.canSetValue` only more confusing,
so let's get rid of it.

* Add `configDisablesSetting` value for Settings

The current magic where this only works for features (but not beta features!)
is, well, magical. And I need more flexibility here.

* Remove redundant levels constant `LEVELS_FEATURE`

I don't know if this was ever intended to have different semantics to
`LEVELS_DEVICE_ONLY_SETTINGS_WITH_CONFIG`, but if it was, those semantics
shuold have been written down. They now seem to be used entirely
interchangably.

src/components/views/settings/E2eAdvancedPanel.tsx

@@ -38,5 +38,5 @@ const E2eAdvancedPanel: React.FC = () => {
 export default E2eAdvancedPanel;
 
 export function isE2eAdvancedPanelPossible(): boolean {
-    return SettingsStore.isEnabled(SETTING_MANUALLY_VERIFY_ALL_SESSIONS);
+    return SettingsStore.canSetValue(SETTING_MANUALLY_VERIFY_ALL_SESSIONS, null, SettingLevel.DEVICE);
 }