Add CORS with allowed origins

[justinr1234]

This adds CORS to the app. It allows you to specify, via environmental variables, which allowed origins you can use. This is useful for a variety of scenarios and is specifically useful in the case of using this server for an API or accessing it from other origins (like using Ionic).

Allow us to pass a list of domains allowed for CORS:

CORS_ALLOWED_URLS=http://localhost:8100,https://mobile.mydomain.com