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.

test/components/views/settings/tabs/user/PreferencesUserSettingsTab-test.tsx

@@ -42,7 +42,6 @@ describe("PreferencesUserSettingsTab", () => {
         beforeEach(() => {
             stubClient();
             jest.spyOn(SettingsStore, "setValue");
-            jest.spyOn(SettingsStore, "canSetValue").mockReturnValue(true);
             jest.spyOn(window, "matchMedia").mockReturnValue({ matches: false } as MediaQueryList);
         });