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/CryptographyPanel.tsx

@@ -62,7 +62,7 @@ export default class CryptographyPanel extends React.Component<IProps, IState> {
         }
 
         let noSendUnverifiedSetting: JSX.Element | undefined;
-        if (SettingsStore.isEnabled("blacklistUnverifiedDevices")) {
+        if (SettingsStore.canSetValue("blacklistUnverifiedDevices", null, SettingLevel.DEVICE)) {
             noSendUnverifiedSetting = (
                 <SettingsFlag
                     name="blacklistUnverifiedDevices"