fix(module:tabs): repeat rendering the TabBarExtra when used nested tabs

[hsuanxyz]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[X] Bugfix
[X] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: #452

What is the new behavior?

added [nzTabBarExtraTemplate] property to make #nzTabBarExtraContent optional

Does this PR introduce a breaking change?

[ ] Yes
[X] No

Other information

[hsuanxyz]

new API

    <nz-tabset>
      <nz-tab>
        <ng-template #nzTabHeading>
          Tab1
        </ng-template>
        <nz-tabset [nzTabBarExtraTemplate]="tabBarExtraContent">
        <!--       ^^^^^^^^^^^^^^^^^^^^^^^                  -->
          <nz-tab>
            <ng-template #nzTabHeading>
              SubTab1
            </ng-template>
            <span>SubContent1</span>
          </nz-tab>
          <nz-tab>
            <ng-template #nzTabHeading>
              SubTab2
            </ng-template>
            <span>SubContent2</span>
          </nz-tab>
          <ng-template  #tabBarExtraContent>
            <i class="ant-tabs-new-tab anticon anticon-plus"></i>
          </ng-template>
        </nz-tabset>
      </nz-tab>
      <nz-tab>
        <ng-template #nzTabHeading>
          Tab2
        </ng-template>
        <span>Content2</span>
      </nz-tab>
    </nz-tabset>

downward compatibility

    <nz-tabset [nzType]="'card'">
      <nz-tab *ngFor="let tab of tabs">
        <ng-template #nzTabHeading>
          <div>
            {{tab.name}}
            <i class="anticon anticon-cross" (click)="closeTab(tab)"></i>
          </div>
        </ng-template>
        <span>Content of {{tab.name}}</span>
      </nz-tab>
      <ng-template #nzTabBarExtraContent>
        <i class="ant-tabs-new-tab anticon anticon-plus"></i>
      </ng-template>
    </nz-tabset>

[coveralls]

Coverage increased (+0.01%) to 41.247% when pulling 6394981 on HsuanXyz:fix-tab into 4efd6f7 on NG-ZORRO:master.