Externalize Neo4j settings into .env file

[rmllr]

To provide a fully automatic demo of the dashboard via Rancher it is necessary to set the Neo4j settings (URL, username, password) externally in a .env file.

By default the dashboard uses the settings of the .env file. However, it should still be possible to customize them on the settings page. Therefore, a checkbox labeled "Customize database settings" is added to the settings page. If the checkbox is enabled the textboxes for the database settings become editable. Details on how to implement this are provided here: https://www.digitalocean.com/community/tutorials/how-to-deploy-a-node-js-and-mongodb-application-with-rancher-on-ubuntu-16-04 Finally, the Neo4j docker files have to be adapted accordingly.