Use devServer Proxy

[gtalarico]

https://github.com/vuejs/vue-cli/blob/dev/docs/cli-service.md#configuring-proxy

Seems like this feature would simplify development server config and possibly aliminate the need for:

flask-vuejs-template/app/api/__init__.py

Lines 19 to 20 in cc5052f

	if not current_app.config['PRODUCTION']:
	response.headers['Access-Control-Allow-Origin'] = '*'

and

flask-vuejs-template/app/client/vue_app/src/backend.js

Line 4 in cc5052f

const API_URL = IS_PRODUCTION ? '/api/' : 'http://localhost:5000/api/'

[gtalarico]

Implemented on new template structure:
https://github.com/gtalarico/flask-vuejs-template/tree/flatten-template-and-yarn