Add an image upload endpoint for uploading pictures

[yunho7687]

make a separated endpoint for uploading pictures.
if possible, allow one question only have one uploaded picture, which means that to delete the previous uploaded one after a new uploading

some useful reference:

1. https://stackoverflow.com/questions/45564130/django-rest-framework-image-upload
2. https://www.django-rest-framework.org/api-guide/fields/#imagefield
3. https://dev.to/thomz/uploading-images-to-django-rest-framework-from-forms-in-react-3jhj

[github-actions]

Branch issue-102-Add_an_image_upload_endpoint_for_uploading_pictures created!