How to make API calls in ngx-admin production server

### Issue type

**I'm submitting a ...** (check one with "x")

* [x] bug report
* [ ] feature request
* [ ] question about the decisions made in the repository

### Issue description

**Current behavior:**
Login in ngx-admin is properly working on dev server. I'm using proxy.config to call the api. But when i deploy that to production, login is not working. I'm getting error as "Login/Email combination is not correct, please try again.".
On debugging, i understand that proxy.config is only for development. So where should I specify API calls in production? How can I achieve this?

**Expected behavior:**
API calls have to work on production.

**Steps to reproduce:**
I followed [Nebular Server Deployment](https://akveo.github.io/nebular/docs/guides/server-deployment#server-deployment)

1. I did `npm run build:prod`
2. Then i run this command to create a server. `npm install -g angular-http-server`
3. Inside dist folder, `angular-http-server`
4. And browse to localhost:8080, shows login page.
But when i tried to login using email and password, shows error.

**Related code:**
N/A

### Other information:

**npm, node, OS, Browser**
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**Angular, Nebular**
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Angular cli: 7.3.5
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