Drag and drop image uploads in markdown editor

[thomasmaclean]

Description of changes

Adds drag and drop image upload support for the Markdown editor. The editor will require a URL endpoint to actually perform the upload. An example API route that could work in the URL parameter (using S3 in this case):

exports.upload = (req, res) => {
	if (!keystone.security.csrf.validate(req)) {
		console.log('CSRF failure');
		return res.apiError(403, 'Your csrf is bad and you should feel bad');
	}
	if (req.files && req.files.file && req.params.path) {
		var storage = new keystone.Storage({
			adapter: require('keystone-storage-adapter-s3'),
			s3: { path: '/my-path' },
		});
		storage.uploadFile(req.files.file, (err, file) => {
			if (err) return res.send({ error: { message: err.message } });
			return res.send(storage.adapter.getFileURL(file));
		});
	} else {
		return res.apiError(403, 'No image selected');
	}
};

Related issues (if any)

Fixes #2071. Based on pull request #2508 by @jacargentina.