Create Tags Management Stubs for Ember

closes #4260

- Adds "tags" route inside of the settings routes.
- Adds this route to router.js
- Links the route from the settings.hbs template
- Adds demo tags html in tag.hbs template
- Adds flag for tagsUI
@novaugust we have an API endpoint (https://github.com/TryGhost/Ghost/issues/3619) for reading configuration values. The idea was to allow ember to read the config upon initialization.  The values were needed for a release but the PR was not ready and therefore the values were added as HTML5 data elements. This line and the mentioned helpers are good to be removed from the codebase :).