Changes to Courses Endpoints

Following changes are required to back-end
- [x] Add a CP category on the `create-article` route
- [x] Rename `topic` to `tag`
- [x] Article must contain multiple tags

The following endpoints need to be created/ modified
- [x] `create-article` - Tags should be passed as an array of items. Each article must be linked to that tag
- [x] `tag-detail/` - GET all articles under the tag
- [x] `tag-list` - GET all tags. Also add new tag wiith POST request
- [x] `category-detail` - GET all tracks under a category