Sidebar scrolling not working properly on iOS Safari

### 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:**
Dear Ngx-Admin Team, we really like your work, and thank You for your contribution and we would like to introduce it in one of our project.
The sidebar menu scrolling is not working properly on iOS Safari. Unfortunately we cannot ignore users with Safari :(

**Expected behavior:**
It would be good to experience the same behavior as on Chrome, Firefox, etc. So when I scroll on sidebar menu it will only scroll on that one, not on the background underneath the sidebar.

**Steps to reproduce:**
Context: ngx-admin demo content, nothing source code modification. Basically this can be experienced when we open Your demo page as well:
When I open the sidebar with the hamburger menu, the sidebar opens it properly, but the touch scroll is "active" on the background underneath thus scrolling the body. When i expand a menu in the sidebar I can finally scroll within the sidebar menus, but when I scroll down to the bottom of the sidebar the Safari elastic feature comes in picture, and again afterwards scrolling on the sidebar is "active" on the background underneath so once again i cannot scroll within the sidebar menus

**Related code:**
On iOS Safari
https://akveo.com/ngx-admin/

### Other information:
IOS 12.1.4.
Safari

**Angular, Nebular**
```
As in the demo:
"@angular/core": "^7.0.3",
"@nebular/theme": "3.1.0",
```

Thank you very much in advance for your assistance.