Add support for uploading images in the body of the post

[alanqchen]

Currently only uploading the feature image to the server is allowed. The only way to add an image is by using an URL. To change this, the following needs to be added:

• Add backend API endpoint to delete an image given the image name in the request URL
• Add parser in the frontend that goes compares the raw old vs new post value and determines which images it needs to delete. (Use regex if possible). All images will be in the standard markdown syntax, and the regex should also check if it contains the API URL.