Add option to customize height and width of the certificate

Currently, the certificates generated have the fixed height and width of an A4 size paper. Users should be able to store height and width in pixels beside each layout.

### To start
- Setup MongoDB and Rabbitmq
- Go to https://account.codingblocks.com and create a client with callback url = "http://localhost:8080/oneauth/callback"
- Export following environment variables
- login: rabbitmq username
- password: rabbitmq password
- DB_URL: mongodb connection string
- CLIENT_ID: oneauth client ID
- CLIENT_SECRET: oneauth client secret

### ToDo
#### Backend
- [ ] In file `src/services/layouts/schema.js` change `UPDATE` and `CREATE` schema to accept height and width in the body
- [ ] In file `src/plugins/worker/handler.js:82` update the pdf document opts to use the height and width from the layout and use the current as default

#### Frontend
- [ ] In file `frontend/components/layouts/LayoutEditor.js` update the component to incorporate height and width input.
- [ ] Also change the ASPECT RATIO of the preview on the right when the user changes height and width.