Type '"xsmall"' is not assignable to type 'NbComponentSize'.

### 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:**
Type '"xsmall"' is not assignable to type 'NbComponentSize'.
I found this issue in these files:
/pages/extra-components/form-inputs/nebular-select/nebular-select.component.html
/pages/extra-components/spinner/spinner-in-tabs/spinner-in-tabs.component.html

**Expected behavior:**
I think the best option is to improve NbComponentSize to allow xsmall. As alternative, change .html to use small.

**Angular, Nebular**
```
ngx-admin and nebular, v8.0.0
```