feat: update to Angular 8, Nebular 4

[nnixaa]

Please read and mark the following check list before creating a pull request (check one with "x"):

• I read and followed the CONTRIBUTING.md guide.
• I read and followed the New Feature Checklist guide.

Short description of what this resolves:

Also closes #2076, #2148

[antonevane]

nebular 4.0.0 has been released

[Gamarsoft]

Hi, when is the update of Ngx-admin (demo / starter-kit) planned ? Thx

[YukiDmZhao]

I git this branch,but can not using,,the style is not normal

[YukiDmZhao]

feat/update-to-8 ,This verson is not working properly. maybe it is style css or others??

[BobCashStory]

Nedd update on this please ! :)

[ivan-penchev]

Shows
WARNING: Nebular Theme: nb-theme() cannot find value for key color-success for theme default
am I doing something wrong with the branch?

[aksolutions95]

Shows Errors like
`ERROR in ./src/app/@theme/layouts/one-column/one-column.layout.scss
Module build failed (from ./node_modules/sass-loader/lib/loader.js):

  /deep/ .main-container {
 ^
  Expected selector.

╷
20 │ /deep/ .main-container{
│ ^
╵
stdin 20:7 @content
node_modules@nebular\theme\styles\core\theming_install.scss 116:7 @content
node_modules@nebular\theme\styles\core\theming_install.scss 97:7 nb-install-component-with-scss-vars()
node_modules@nebular\theme\styles\core\theming_install.scss 115:5 nb-install-component()
stdin 5:1 root stylesheet`

with below warnings
Nebular Theme: nb-theme()cannot find value for keybtn-border-radiusfor themecosmic node_modules\@nebular\theme\styles\core\theming\_get-value.scss 59:5 nb-theme() src\app\@theme\styles\bootstrap-rtl.scss 24:37 @content node_modules\@nebular\theme\styles\core\_mixins.scss 312:5 @content node_modules\@nebular\theme\styles\core\_mixins.scss 306:5 _prepend-with-selector() node_modules\@nebular\theme\styles\core\_mixins.scss 311:3 nb-ltr() src\app\@theme\styles\bootstrap-rtl.scss 20:7 bootstrap-rtl() stdin 33:3 @content node_modules\@nebular\theme\styles\core\theming\_install.scss 149:7 @content node_modules\@nebular\theme\styles\core\theming\_install.scss 135:7 nb-install-global-with-scss-vars() node_modules\@nebular\theme\styles\core\theming\_install.scss 148:5 nb-install() stdin 19:1 root stylesheet

Need update of ngx-admin for Angular 8.