Sidebar partially shown under the header (3.6.2)

### Issue type

**I'm submitting a ...**

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**
In version 3.6.2, the sidebar is shown at a different height (i.e. with a different `top` value in the CSS).
This causes the sidebar to partially overlap with the header:

This is caused by the following line of code:
https://github.com/akveo/nebular/blob/e5a04b6fb2b3cdf6e76f8294cbeaffa37427e8a1/src/framework/theme/components/layout/_layout.component.theme.scss#L161
Which calculates the top to be 4.25em (for the default theme), not taking into account the additional 2rem margin for between the header and the content.

**Expected behavior:**
The sidebar should have some margin with the header, and not overlap (like it used to in 3.6.0 and earlier versions)

**Steps to reproduce:**

Just add any standard sidebar:
https://stackblitz.com/edit/nebular-sidebar-issue

**Related code:**

Please refer to my stackblitz:
https://stackblitz.com/edit/nebular-sidebar-issue

### Other information:

**npm, node, OS, Browser**
```
Node 10.15.3
NPM 6.10.1
OS: Windows 10
Browser: tested in Chrome/Firefox
```

**Angular, Nebular**
```
Angular: 8.1.2
@nebular/theme: 3.6.2 | 3.6.1
```
The issue doesn't happen in Nebular `3.6.0`, so my current workaround is to enforce that version of `@nebular/theme`